I think its basically the same as the 305 (which I've got). The navigation isn't great its just a track line on a bank map so I wouldn't rely on it but it has opened the doors (so as to speak) to new places

The biggest issue for me is when there are two or more roads in one direction and I can't say for certain which one it is, until I'm half way up it and I sometimes have to backtrack which I don't mind.
The main advantage of the 305 gps to me is the recording of stats, spot speeds, average speeds, gradients, elevations, heart rate, cadence, power (if you also buy a meter) and I believe the 500 also records the temperature. If you value stats yes IMO its worth it but if you intend to use it purely for navigation its not.
Edit: If you get one for creating routes I find
ridewithgps the best, its quick and makes useful .tcx files and simplified .gpx route files which the system can handle.