I have an Garmin Vista HCx and quite happy with it. However there are some quirks, or rather self induced design features, that can be frsutrating - not deal breaking by any stretch, but frustrating neverthe less. Note that some of this could be problems with the user (me
) not spending enough time to really learn the interface and different buttons to press at different times/places in the interface.
To comment on your points:
- I want a GPS on which I can enter my route in advance. I understand you can download the GPS data for the ACA routes from their website. That would be a must, because there are several ACA routes I want to try in the future.
Can't imagine a GPS device that doesn't offer this. Question is how to do the route on your PC, using the MFR's own product like Map Source (in Garmin's case) or other commercial map software. Or go the "free" way and figure how to DL the maps onto the device. I use Map$ource and the normal transfer setup via the USB cable.
- I want to be able to change my route on the road. In other words, I don't want to be dependent on my home computer to enter routes.
hmmm, depends what you really mean by change the route. You can certainly choose to go off the calculated route between two waypoints, the GPS units will just recalc the route dynamically as you go. This presumes to some extent that you have another way to get from point A to point B in mind, and that it actually can be done. I have just "freelanced" an alternate route many times, but bear in mind that in unknown territory you may just end up going in a big circle or the dreaded "make U-turn" I found this technique really usefull to avoid traffic or some imposing looking hill or just to go down a scenic looking strech of road.
If you really want to make up a new routes that encompasses many waypoints, that can be done via the pretty primative GPS interface, and is equivalent to just scrolling around on a 2" screen and clicking waypoints as you go - not a particularilly fun time let me assure you, but definetly do-able with some patience.
- I want to know where restaurants, grocery stores, campgrounds, motels, historical attractions, bike shops, etc. are.
This is a given on most GPS's (it is on mine for sure) and is one of the unmitagated joys of using a GPS for a tour. There are some gotchas though, like the store might of closed, bike shops are mainly motorbike shops etc... but I used this feature a lot 
. I would get close to the end of my biking day and start looking for food/resturants followed by lodging/camp grounds
- I want it to work under a tree canopy. (I had an Etrex I bought for my kayak. When I took it backpacking it had a lot of trouble receiving a signal in the woods.)
Tis my understanding that the Vista HCx has enhanced satelite reception ability above some of its cousins, howvere I never used it in a deep woods environment. I've biked through forrests though and never had a problem. YMMV
- I'd like to have an altimeter. I'd like to be able to download my routes onto my computer after I get home and have a display of the elevation. This isn't crucial, but it would be fun and interesting.
Yep, my HCx has the altimeter and yes it was fun and interesting. Just remember to reset it at the corrects points so you have a good idea of max altitude for a particular leg of your tour as opposed to the ride you did last week with Uncle Bill.
IMHO it would be best to compliment the GPS with a full set of paper routes - I did and used both together. A 2" screen it just too hard to help you conceptulize where you are going and to read the topology (assuming you have a map w/topology). Battery consumption is an important issue too. I generally saw about 3 days of use before I had to get new batteries (2 x AA). I used Lithium mostly - def better capacity, but not cheap. You might be able to use a solar charger (or even wind), I would think about that option for a really long trip. One last but very important note.
AFAIK, no current GPS unit has the ability to route based on grade info or total climbing required between point A and point B for example