Although I've used Logitech mice for years, I'm a bit disappointed at their recent offerings. My all-time favorite is the now discontinued wireless MX-700, with two thumb buttons a small button before and after the wheel and another button a little farther behind the wheel.
Additional features would be great, but it seems Logitech has only RETREATED from this 8-button configuration.
I use the thumb buttons, of course, for page back/forward, but also use the additional buttons for COPY, PASTE, and Shift-Delete. With Firefox's Autocopy extension installed, I save tremendous time navigating sites and posting in forums with this configuration.
Newer mice use different software that doesn't seem as fast or as configurable. Nor do they have 8 completely programmable buttons if you don't want to manually modify the XML files!!!
Bring back the MX-700! Give IT a tilt wheel and let me configure -everything-! :)