I used to have a 500 as well, and replaced it when it developed the same issue as yours. Have since upgraded to a 700 because I decided that I wanted as few wires with my system as possible. I really like it - it can be run wirelessly until the batteries die (5ish hours), and then if you need it to roll longer you can either replace the AA it uses (comes with an eneloop, and I have a stock of those so that's what I do), or just plug it in and use it in a wired config. It also has the same basic layout and functions as the 500.
I considered the 600 but it just had too many buttons that looked uncomfortable and that I would never use.
ETA: One that I almost went with was the G602, which is purely wireless but has an extremely long battery life. You might check that one out. The main thing that killed it for me was that it doesn't have the free scrolling mousewheel option like the 500 and 700 do.