I've been using the Logitech G15 keyboard and G6 mouse at home and really like both.
I've had a lot of Microsoft InteliMouse Explorer 4.0 'buttons' start to go south this year, sometimes requiring two and three clicks before registering a single-click... I'm really suprised at the number of failures, but all things considered they have been in place for 5 - 6 years now.
I'm also in agreement with the thumb button... that's really why I chose the 4000 model since it was one of the few that had a thumb button.
Cheers!