Mine is the logitech MX revolution.
Once you try the scrollwheel, you cannot go back to any other mouse. It has no clicks, you can roll it in a 'free' way.
Same here. I love the MX Revolution - for the main scrollwheel, the shape and feel, and for the thumbwheel. Although for the main wheel I still use the click mode, can't get used to the free scrolling.
After a year or so, my MX has developed a problem though: every so often (like, once a minute during heavy mousing) a single left click generates a doubleclick instead. The mechanical contact thingie is wearing out, I suppose. This is hugely aggravating: click a delete button on a toolbar and a program performs two deletes instead of just one. Click the top-right (x) to exit an app, and exit two.
So I've been looking for a replacement, and it looks like the only candidate is another MX. I can no longer live without the thumbwheel! The two thumb buttons are also quite comfortable - I have them set up as zoom in and out. In any other mouse I've tried any additional buttons are positioned wrong somehow, out of comfortable reach - either too close to the middle of the mouse body, or too close to the front edge.
For my wife I recently got a Logitech VX Nano Cordless - looks like a mini version of the MX, sans the thumbwheel. She says it's the best mouse she's ever used. (Mouser note: this one uses AAA batterries, no recharging.)