Comfortable as a worn-in pair of shoes
I must say the first time I pulled the mouse out of the box and started tooling around in IE, Firefox, Photoshop, Flash, Word, Excel, and Visio the MX Revolution felt as comfortable as a worn-in pair of shoes. The "shifting" mechanism inside the mouse's scroll wheel (switching between free wheel and click-to-click modes) is the best innovation in input devices since the scroll wheel itself.

First, some backstory
I spend about 12 hours a day working in Web design and development so top quality tools are very important. For me price doesn't matter if it is truly a top-quality tool. I spend a lot more time interacting with a mouse than I do with any other piece of technology.

My old (work) mouse was a Microsoft Intellimouse Explorer 2.0, the first mouse to feature the left/right tilt wheel. I literally had worn out the mouse wheel mechanism from so much "wheeling." The Logitech implementation with the MX Revolution is better as there is less travel in the tilt wheel (side to side).

Good software for configuration
But with all the kudos for ergonomics, style, good battery life, good response and accuracy, what really brings it together is the software. Logitech (IMHO) has historically writted terrible software for configuring devices (I have the wireless desktop elite, which is awful), but the SetPoint software is suprisingly well designed and easy to use. Not only are all the mouse buttons customizeable, you can even customize them for individual applications! So for Web browsers I have the wheel mode set to free wheel, and Photoshop I have it set to click-wheel mode. So when I switch from web browsing to Photoshop it automatically 'clicks' into the right mode. You can feel the mechanism 'switch' inside the mouse. It gives the sensation that it's working for you.

Thumb wheel not useful but back button is
The side wheel isn't that useful for me, my thumb doesn't reach it for switching programs, so I just configured the "back" button to open the program switcher. Logitech implemented their own version of the Windows ALT+TAB dialog, which shows a vertical list of open programs with their titles instead of just a bunch of icons (which you have to "TAB" over to get the title), which I feel is much easier to use because I often have multple word documents and Web pages open so I like to see all of the titles at a glance. This can save me a few seconds each time I switch programs.

Hope it lasts
The only other thing remains to be seen if the mouse lasts and holds up against the hours of work I will put against it. I have heard these things break easily or defects appear within about 2 weeks. I have been using mine for a week with no problems.

The reason I write such a glowing review is that I use input devices a lot to earn my living, and am very happy when an innovation comes out on the market that makes my job easier. Support innovation, by using it!