I will also confirm that which n3rrd has said. I have had my M570 for a month or two now. When I first used it, it was quite. After about two days, it was decided to squeek. I never bothered with it. Now after a few months, the scroll wheel has no squeek. Initially the scroll wheel may have felt a little bit gritty as well. That is also gone, and the wheel is much smoother now.
With all that said, I really like this trackball. I originally thought I wanted the finger ball, but I find that the thumb ball is ideal. The buttons and scroll wheel feel right where they belong, especially coming from a mouse previously. Click dragging is a breeze. Clicking without moving is also no problem. I also love the fact that the trackball is wireless. I use a keyboard tray that slides in and out. I appreciate not having an extra cord to worry about hanging up. I also do not need to worry about routing the cord either. I wish that my RK-9000 came wireless as well.
My only complaint with the ball is that I am very sensitive to any friction. I find that I do need to clean the ball and rollers somewhat frequently. I am not sure if the stainless rollers would be better or not in that respect. Cleaning the ball and rollers (little tiny balls) is very easy though, and I will do that before going back to a mouse.