I've bought three of these trackballs. Logitech's is by far the best - UNTIL - something causes the ball itself to roll irregularly and in herky jerky motions. I've use compressed air, wiped out the cavity that holds the ball, washed the ball. And nothing restores its smoothness.
Perhaps I've overlooked some remedy to this problem.
I tried this mouse out for a couple of days and a few things you need to be aware of. The tracking of the mouse through bluetooth is a little slow to respond. You will find yourself moving the mouse more than normal no matter how you change your settings. I ignored this when reading the reviews hoping that it wasn't as bad as they made it out to be, but it isn't that bad but can be annoying. The features to the mouse are very nice!!!, however I found the two finger moves were rather difficult to do. It is also small. If you have larger hands, it can feel cramped after long periods of use. If there was a bigger mouse with more response it would be an absolute 5 in my book. The only reason I returned it was for those two reasons only.