I personally use logitech marble mouse. I have tried many other trackballs but I have not found any that were more accurate and comfortable to use. As a bonus the unit is completely symmetrical so you can use it with either hand (I have three on the left side and three on the right side of my XKeys to best utilize space). It took awhile to learn how to use a trackball with both my hands but it was well worth it. The unit costs about $20 so it is very affordable:

http://www.newegg.com/Product/Produc...marble%2bmouse

The only downside of this device is a lack of scroll wheel. I also used the wireless trackman from logitech and while not bad it is not as good as the marble mouse (and costs $50 to boot). But if you have to have a scroll wheel it fits the bill:

http://www.newegg.com/Product/Produc...82E16826104125

I tried a bunch of top end products from both kensingtom and micro$oft (about the only other two companies making trackballs) but they were not as good as the logitech ones.

I also used the logitech thumb operated trackball:

http://www.newegg.com/Product/Produc...82E16826104156

and while definitely usable your thumb is simply NOT as precise and fast as your other fingers. The same goes for your wrists when using a mouse.

Ziran