I'm about ready to replace my 20 year old Nishiki with a brand new Cannondale Synapse Alloy 6. I will be getting a bike computer of some sort. I would really like a GPS unit made for biking, or perhaps general outdoor use. I'm interested in comments from people who have used GPS units.
I won't ever go back to not riding with a non GPS bike computer.
I've used the Garmin Edge 500 for almost a year and have used a Garmin wrist watch to record before that. Having the computer on the stem is much better than my wrist.
I have never used a bike GPS for navigation, just as a bike computer to show speed, distance, cadence, HR and power (ANT+ from my Power Meter).
I love analyzing my ride after, looking at the map, seeing where I struggled and where I really hit it hard. I think this is the best part of a GPS bike computer as it will 'record' your whole ride. You can upload a map to your Facebook, see how much elevation you rode, compare rides with friends who rode that same route before.
The software (Training Center) that comes with the Garmin devices is not all that great but there are many alternatives, such as Garmin's online equivalent (Garmin Connect). I use Strava.com almost exclusively now, but have used TrainingPeaks.com as well as downloaded software on my computer called SportTracks.
I think the Garmin Edge 500 is a great value right in the middle of their offerings, you can go a bit cheaper (into low $100's for the Edge 200) or full color with maps (Edge 800). I have never run the extra speed/cadence sensor, just relying on the GPS for speed/distance, my riding buddies do like their speed/cadence sensors, as you can then use it on your trainer in the garage.
I bought a waterproof case/mount for my Android phone and I think it's awesome. There are a stack of apps in both the Android market and the iTunes store that track everything using GPS and let you connect to monitoring devices using Blutooth. Only issue is it eats the battery like there is no tomorrow to use the GPS I bought a bigger battery and I can get 3-4 hours of cycling as well having the phone to use all day.
I know it's not what you asked but it's an alternative option which is cheaper if you already have a smart phone.
You've got a small handful of choices for GPS enabled bike computers.
Hope this helps!