I've got the 510.  I've also had a 500 and the timex version.  I like all the garmin stuff. The reason I went to the 510 is because I needed more than a 3 bike memory.  Both were easy to use although I do think the 510's GPS is more accurate looking at when I ride multiple laps and how it overlays them.

I highly recommend one if you are a data freek. The biggest thing is that if you want accurate mileage, then you need a speed cadence sensor and it needs to be properly calibrated.  They don't cost that much but with out it the gps will short you every time on the mountain bike.  Road bike doesn't seem to be a issue as your not doing tight switchbacks.

Biggest concern is loosing it or braking it in a crash.  But if you are riding around with an Iphone, I would rather loose the garmin than the phone.

I picked up the 510 30 days ago and am really enjoying it. Does anyone have experience with the Live Tracking feature? It doesn't appear to be tracking 'live' rather refreshing my stats/ position in 60 second intervals. (so says my wife) This is the case on the CPU and handheld. 

Any thoughts?

If you are only doing MTB, 510 is the way to go.  510 has GLONASS gps, while 500 does not... 510 is much more accurate, particularly in tree cover.  If you're segment hunting, the 500 will sometimes have so much gps drift that it will miss segments.  500 will also struggle with start/stop if you are in tree cover - you will get an annoying "beep beep" more often than necessary.