I feel like I should post in defense of the 500
I had a hard time deciding between it ant the 800. The 500 may give you more than you want, but the GPS-linked odometer, ability to put in breadcrumb maps and use of the training center to look at your elevation profile, distances, pace, etc, is really nice.
I really like the 500 for its HR and cadence features, which I use on 3 different bikes easily as well. The other features are a bonus, to me.
I use the breadcrumb maps to find my way following my SO, who's training for a 12 hr TT. Since I'm not, I put his planned route in my 500, and I sort of follow him and try and plan to finish his ~60 mile ride with him while riding ~20 miles myself. So far that's worked pretty well. We missed each other once, when I stopped for a latte