I have a M570 (Wireless trackball, thumb ball, as opposed to centered). I love it. The one thing is that it loses sensitivity when gunk starts to build up - it

I have a M570 (Wireless trackball, thumb ball, as opposed to centered). I love it. The one thing is that it loses sensitivity when gunk starts to build up - it becomes harder to move the mouse a pixel or two. But that's easily fixed by popping out the ball and cleaning it. The other issue is that I've had to replace the primary mouse button - the microswitch died.