I've used my Edge 800 on a tour in the Netherlands and on hundreds of shorter rides. I find it's quite useful, with some caveats. Here's a lengthy review, which I originally wrote as a post to a mailing list and that was subsequently revised for publication on sheldonbrown.com, which John S. Allen is now maintaining. He hasn't had time to post it there yet. By the way, the 800 does not have voice prompts; it beeps, but you have to glance at the screen to see turn instructions.

Comments on the Garmin Edge 800
by Brian Ogilvie (http://www.pobox.com/~ogilvie)
Version 1.1, 2011-11-28

The Garmin Edge 800 is a GPS-enabled cycle computer that also provides mapping and routing functions. It is not quite a full-fledged handheld GPS unit, lacking a digital compass and a few other features. But it comes very close and for road cyclists it can replace a GPS and a cycle computer. (Note: except for the mapping functions, I think most of these comments apply to Garmin's GPS-enabled Edge 500 cycle computer too.)

The Edge 800 provides a wide range of information about your ride: instantaneous speed, average speed, distance, lap distance, total time, lap time, elapsed time (total time including stops), altitude, grade, total elevation climbed/descended, temperature, and probably a few things I'm forgetting. You can create (only!) 3 bike profiles [update, 2012-03-18: the most recent firmware allows 5 profiles], each of which has its own weight and odometer. With optional heart rate and speed/cadence sensors, the Edge 800 will also display your instantaneous heart rate, average heart rate, speed, and cadence. It works with ANT+ power monitors, too; unlike some of Garmin's other GPS-based exercise monitors, it can take speed-only and cadence-only sensors, as long as they are ANT+ units. If you need more data than this, you are probably on a stationary cycle in a lab.

You can set up three different screens to display as much or as little of this information as you wish. The fewer fields you show, the larger the display is. There's also a map screen, and if you're following a course, a course time/distance screen and any turn-by-turn directions. The 800 also shows an elevation profile of where you have ridden, and if you are following a pre-programmed course that includes elevation data, it will also show you what lies ahead. It comes pre-loaded with several workouts, and you can define your own. You can use a variety of mapping programs to define courses, which you can then follow on the bike. With the right maps loaded, you can also use the unit for routing and guidance, like an automobile GPS--but no digital voice, just a soft beep alerting you to turn information on the screen.

Pros: The Edge 800 is easy to switch between bikes, since there are no sensors to install or move (unless you use optional speed, cadence, and power sensors). The quarter-turn mount lets you attach and remove the computer quickly, but it also holds the computer securely to the bike. The screen offers large numbers, with customizable display of the fields that most interest you. You can set it to auto-start when you take off, and auto-stop when you stop moving, or if you're doing randonneuring you can set it so that its averages include stops. The deluxe heart rate monitor strap is quite comfortable (it's elastic cloth with a couple of small plastic patches; the transmitter snaps off so that the strap can be hand washed).

If you're into counting calories, the Edge 800 uses several algorithms to estimate how many you've used, depending on how much data it has available. It uses your weight, your bike's weight (based on the active bike profile), distance, and speed; if you have the heart rate sensor, it also factors in your actual heart rate and your maximum and resting rates; and if you have the power sensor, it takes that into account too. If you're used to calorie estimates from simple heart rate monitors, you'll probably find that the Edge 800 gives a lower number, which is probably much more accurate!

Cons: Accuracy varies depending on location. Garmin's GPS receivers are now accurate to within about 10 feet in ideal conditions. If you are in a city or a forest, accuracy drops somewhat. That means that the speed displayed on the computer will fluctuate. Under open skies, my 800 fluctuates about 1.5 mph up and down. In heavily wooded areas, the fluctuation can be more like 5-6 mph, depending on density of trees, the nature of the terrain, and cloud cover. The speedometer tends to read low in those conditions: I can be going along at about 15 mph and the speedometer might read 8 or 9 mph.

Since I rarely care about my immediate speed, the fluctuations don't bother me. If they bother you, you can get an optional speed and cadence sensor from Garmin, or one of the ANT+ speed sensors from Bontrager or another company. Obviously you'd need one of the sensors for every bike you wanted it on. You then use the Edge's GPS features to calibrate the sensor (no rollouts necessary!); the Edge then uses the sensor to give you your speed, but it still uses GPS to track your ride. Note: you can't use both the speed and cadence features of the Garmin sensor on a Friday, because the cranks are too far forward of the rear wheel; it's possible to modify the sensor if you don't mind cutting it in two and soldering an extra length of wire. Or you can buy a separate ANT+ cadence sensor; the 500 and 800 work with multiple sensors. I've seen multiple complaints on bike forums that the Garmin sensor's reed switch for the speed function is prone to breaking, but those who have contacted Garmin USA report that their units have been replaced under warranty.

Other observations: Garmin's GPS sensor locks on fairly quickly, usually within 20 seconds of being turned on, unless you're inside. It saves your last location and starts its search from that. Hence, if you travel with the unit turned off, it can take up to a couple minutes to find your location because it has to do a more thorough search for the satellites. I usually turn my Edge on before loading water bottles and verifying tire pressure and chain condition; by the time I've done that it's ready to roll. If I'm in a new city, I turn it on while getting dressed and set it on a windowsill so it can get a fix on the position.

Some people have complained about the resolution of the Edge 800's touchscreen, which is lower than that of its predecessor, the Edge 705. Not having used a 705, I think the 800's screen is fine. The touchscreen is easy to use and can be locked with the power button (handy if you're carrying the unit in a pocket). The backlight is adequate to good. The one difficulty I've encountered is that the light blue color that the unit uses to indicate a course on the map can be hard to see, especially if you're wearing amber sunglasses.

GPS routing is not as good as on an auto unit; my guess is that the Edge 800's microprocessor is optimized for low power use, not processing power. It works best over short distances. If you deviate from the calcuated route, it will recalculate, but not necessarily very well. And it is only as good as the maps; see the note at the bottom on mapping. If you are following a pre-programmed course that is set up as a GPX track or as a TCX file (Garmin's proprietary format), any turn directions will depend on what you programmed into the course file.

Because GPS and backlighting use a lot more power than a simple cyclocomputer, battery life is measured in tens of hours (Garmin claims up to 15 hours, but I find that 12-14 hours is more realistic; the Edge 500 is rated for 18 hours). That means you need to remember to recharge your computer regularly. It's not like a cyclocomputer where you change batteries once or twice each season. You can buy a cheap USB battery as a backup power source. And it will charge from your computer when you connect it to download your route data; if I've been out for a short ride (under 2 hours), half an hour of being connected to the computer will recharge it fully.

The Garmin Training Center software, into which you can download your data for storage and analysis, is fairly basic, at least on the Macintosh. I use it mostly to archive data in case my unit conks out. Garmin's free Connect website is somewhat more useful; third-party sites such as Strava, Training Peaks, MapMyRide, and (just getting started) RideWith GPS can also download and analyze data from the Edge.

I like my 800 and feel like I have gotten my money's worth from it. I use the mapping feature while touring or following unfamiliar routes. It's not quite a full-fledged GPS unit, though: for example, it doesn't have a real digital compass in it. If you don't need the mapping features, the 500 would be a good choice.

DC Rainmaker has posted in-depth reviews of these and other Garmin GPS products on his website at: http://www.dcrainmaker.com/

The Garmin forum for the Edge 800 is also useful, keeping in mind that most people go to the forum only when they have a problem, and that the vibe therefore tends to be negative:
https://forums.garmin.com/forumdisplay.php?f=245

NOTE ON MAPPING: Garmin's commercially available road maps, which are designed for use by motor vehicles, lack most bicycle paths, rail trails, multi-use paths, and other facilities that are open to bikes but not to cars. In many parts of the world, free maps based on the OpenStreetMap.org project work better, though there is something of a learning curve to using the most comprehensive of them. Some people, especially in Europe, have created OSM-based maps that are optimized for cycling. The VeloMap.org maps are the most comprehensive but the last time I tried them, they took a fair amount of work to compile and load. I think their creator now allows you to subscribe to his site and get precompiled maps. Other maps are better; in particular, the FietsMap of the Benelux countries, and the OSM-based bike map of Switzerland, are both excellent, free solutions if you're cycling in those countries. If you're going to rely on OSM-based maps, I suggest that you compare the OSM map of the area where you'll be with commercially produced maps (such as Google's or Bing's maps) to see how the coverage compares. In major cities, OSM is usually great; in the countryside it can be iffy, especially in countries where national map datasets are copyrighted.

Information on using OpenStreetMap-based maps with Garmin products is at:
http://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin

Websites where you can upload and analyze data from your Edge include:
http://connect.garmin.com/
http://www.strava.com/
http://www.trainingpeaks.com/
http://www.mapmyride.com/
http://ridewithgps.com/

NO CONFLICT OF INTEREST: I have no financial interest in Garmin that I'm aware of, though it is possible that one of my mutual funds has Garmin shares. My Edge 800 was a personal purchase with my own funds.