Personally I've never really liked thumb-trackball mice, I used plenty of them in the old days but found them to be unreliable. But my boyfriend bought one and since I use his computer a lot (especially for gaming) I was forced to get used to it. If I had to pick a thumb-trackball mouse, I'd definitely choose this one - not only is it a trackball, but it's also optical, but I'll get to that a little later. Microsoft has given the trackball an amazing comeback.

Feel (AKA Put Your Hand On My Mousey)
The first obvious difference with this mouse is that you don't move it. It sits in one place and all you have to move is your thumb. Whether this is a good thing or bad thing I'm not quite sure.
It's a pretty big mouse and fits most any hand. The shape of it is very comfortable, and resting your hand on it feels completely natural. Because of its shape it can be used only by right handed people. The feel of this mouse is very nice. It's very ergonomic and doesn't put a lot of strain on your hand.

IntelliEye??
I was just as baffled as you when I read this on the box. These companies seem to always be making up crazy new "technologies" and not telling us. So it goes with Microsoft's use of "IntelliEye Optical technology" used on this mouse. Basically it's just that they merged together trackballs and optical mice into one. No longer does the trackball have mechanical parts, gears & rollers, but the trackball just sits in it's cute little hole and is sensed by an optical light (you know? the bright red one?) This is a GREAT feature, because it works much smoother and because it does not suffer the cleaning and breaking of normal trackballs. Onward Optical Soldiers!

Other Great Features
This Mouse has not two, not three, but FIVE buttons. Even better - all five buttons are programmable via the driver. The two side buttons, which are small and don't get in the way of normal mouse use, can be used to go forward and backward on the internet or can be programmed to other uses (copying, pasting, printing.) This gives the mouse many versatile uses depending on the user.
One thing I love about new mice is the USB connection. Ta-da, Plug N Play! This makes it so much easier to plug in, use, and takes up less ports (since USB ports can be multiplied cheaply via a hub.)
It seems that Microsoft has gone the way of Mac. This mouse is both PC and Mac compatible. Way to go Microsoft.
Aside from all this, the mouse is Good Looking. It looks like companies have left the standard-grey road behind and picked a more well-designed path. The mouse is black with a cool red/black patterned ball which matches the red light.

But What About the Gamers?
I was a little afraid to try gaming with this mouse, because it looked like a really bad way to navigate. On the contrary, once I got used to it I found that it is much faster than a normal hand-held mouse, you are able to react a lot faster with your thumb than you are with your entire hand. The downside is that you get slower precision because it's hard to stop your thumb quick enough in one spot - which tends to be bad if you're playing a first-person shooter and need to be right on target. I would say it is a fine mouse to use for gaming, but use what you're most comfortable with.

The Mousing Device Choice
I love how Epinions calls the category "Mousing Devices"...ok, so should you buy this mouse? Well, if you like trackball mice YES. This is the superb and best Trackball mouse I have used. The fact that it's optical as well is something that makes it so great. If you're looking for something very ergonomic YES - this is one of the most comfortable mice I've ever used, and puts very little strain on the wrist. Personally, I will stick with my handheld mice, but overall this is a great mouse, I see nothing wrong with it, and I would highly suggest it.