I'm also looking for a new mouse so I hope help comes soon. I'm a Logitech man looking to buy a new backup mouse for my 4.5 year old MX500. No problems with this yet, only thing was the left button stopped responding. But I took it apart and fixed it and it's as good as new!
I would buy another MX500 in a second, if they still made them. Maybe the 518? Ugly as hell, but good reviews.