optical mice

I had a Logtech Trackman Marble FX for about 8 years before the right mouse button stopped working... But that's another story. You can read my review about that one if you want. I'll go straight to the pros and cons:

Pros:
Ergonomic shape, nice large ball operated with your fingers and thumb.
4 mouse buttons and a clickable scroll wheel, perfect for gamers. All buttons are reassignable to perform any function, or to do a variety of functions within windows. Note that the bundled software calls middle click "autoscroll," and buttons 4 and 5 are called the back and forward web browser navigation buttons. Good for your average user, but a techie like me likes the buttons to be called what they really are.
It's wired, unlike the Logitech version. Wireless products are a solution looking for a problem for me, and I don't like shelling out the extra dough.
You can reassign buttons to do different functions on a per-program basis. For games I have it assigned to the default button setup so that the game recognizes all the buttons as mouse1, mouse2, etc.

Cons:
Very imprecise at high sensitivity settings. The cursor will actually move in steps of several pixels. This might be because the dots on the ball are rather large, but not certain. The solution for those who like high sensitivity settings is to set it for medium sensitivity and turn on low acceleration. The non-linearity that results makes you feel a bit disconnected from the onscreen cursor, however.

Not pros and cons, just observations:

By default, the left-right-middle mouse buttons are operated by your thumb instead of your fingers, unlike a regular mouse. This can be reassigned, I've reassigned the far right button to right click because it's a bit more intuitive, and I have a button bound to shift so I can use it and the scroll wheel for forward-back navigation in my web browser.
It takes a few days to really get used to, like the transition from a straight keyboard to a "natural" one.
It looks like there's three red LEDs inside, but only one is actually near a sensor, the other two are just there to make it glow an ominous red.

I LOVE this mouse! I've had it for a few months now and I don't regret it. The programmable buttons are excellent. I have the far right one as right-click button, the second to the right button as the forward navigation button, the far left button as the left-click button and the second to the left button as the back navigation button. This has helped me spread out my hand and allows my pointer finger to be the one I use for the back navigation button (although I can also use my thumb).

I use this trackball because I have limited desk space.

First off, many of the problems people are writing in this space are PICNIC- 'problem in chair not in computer', or in this case, the trackball.

1-Clean your mouse. Oil from your hands and dust in the air will collect inside. After a month, running your finger inside the rollerballs can be done as an afterthought and takes two seconds.
2-Inproperly assigned keys? Give me a break. You can change them yourself, or, even better, get used to them. They make a ton of sense. I switch between this trackball and a Microsoft optical mouse at work without thinking about it. The back-forward buttons on the right side of the unit are perfect for web browsing.
3-Not ergonomic? I am 6'4 with huge hands, and it is no problem for me. On the other hand, my wife has petite hands and uses this trackball without complaint. If this mouse doesn't fit your hand, then I question your own sitting position/desktop arrangement.

This trackball is ideal for limited desktop space. The trackball is large and gives you plenty of control over motion. I use it all the time for business applications, web browsing, adobe photoshop, and also for gaming purposes. As with ANY pointing device, whether it be stationary or not, there is a little adjustment period, for me it was a few web pages. But if you're looking for a full-featured trackball, this is the way to go. 5 stars.