If you don't need maps you might consider a Garmin Edge 500. The price just dropped to $150 at REI and several other places. I just ordered one to replace my trusty Forerunner 305 (a $43 REI garage sale find).

I wanted something with a barometric altimeter that is compatible with power meters, and I didn't need a 510 or 810 for the phone-connected live tracking or the map navigation. I always carry an Android phone and I can use any number of apps if I need a map, since I typically ride where I have cell service. For live tracking, I just started using the Glympse app. So far it seems to work well without eating up too much phone battery.

Newer Samsung phones like the Galaxy S4 have ANT+ capability built in, which allows you to just use your phone as your GPS-based bike computer. I've been using my S4 with the ipBike app as my main bike computer lately. With ipBike you can collect your GPS readings from you phone GPS (not as accurate as a Garmin but not bad) as well as your ANT+ sensor data (heart rate, speed, cadence, power). You can save each ride's data as a .tcx or .fit file and upload them to Strava, Training Peaks, etc. directly from the app. You don't get temperature or altimeter elevation readings, but you can typically get corrected elevations once you upload your data.

The big downside to using your phone as your bike computer is the battery drain - not a problem if you only do short rides, but on longer rides a dead battery leaves you without phone or computer. That's the main reason I ordered the 500 - to save the phone for communication and emergencies.

Another downside is that your phone can take some hard jolts when mounted on your bike, and you have to worry about crashes. I have jarred loose the SIM card in my phone 3-4 times, but it usually takes a speed bump to cause that.

One last thing - the 510 uses GPS and GLONASS (Russian) satellites, while the 500 only uses GPS satellites. I've read in one blog that the 510 is much more accurate for mtn biking (where trails are narrower) and in mountainous/dense tree cover/urban canyon environments. I'd like to know if anyone here has done a side-by-side comparison of the 500 and 510 to see if the GPS accuracy difference is significant.