the 520, quite frankly sucks. I had it for about a week, and did not like the ergonimcs, the squeeky buttons and little other things about it. It used to lock up on me once in a while even after firmware updates etc.
I returned it and I opted for the 880 for $179 and Ecost (free shipping)
This is going to be a something I'll have forever so I figured I'd get the 880.
I can't comment on the much 600 sereies, but the bigger LCD on the 880 is a pretty nice feature you can actually read the other funcitons of your devices where as on the 520 with the smaller LCD, the button lables were cut off and it was kind of painfull to navigate through that