I have the 705 and it has worked very well except for one ride where the magnet got moved when I had the bike on a rack. The GPS is not perfect, but if you combine it with the wheel and cadence sensor, its very good and very reliable. I dont try to use it as an odometer for my bike because I dont record every single ride I do. I know exactly how long my commutes are, so I just log them on the computer every few days.
The calorie numbers are pretty useless though. If you just want to see how much they go up and down with different riding, it might be useful, but the actual numbers seem way overinflated. That seems to be the general consensus from what I've read from others as well.
My favorite thing about the Garmin (including the 705), is the ability to plan a ride in advance and download the gpx file to your device and follow it. If you ever do long rides in areas where you are not familiar, this is very handy. I have never used the navigation stuff the 705 has and would probably have been just fine with the 500, but I stumbled on a really good deal for the 705.