The M570 improves on the older Logitech Wireless Trackball in a number of ways. The USB transceiver is now a micro sized unit and is "Unified" so it can work with other "Unified" Logitech wireless devices as well. The trackball now has additional "back/forward" buttons although they are located in an odd location that requires you to use your index fingers. Admittedly, I am used to using my thumb for those buttons on a mouse but there's nowhere to place them with the big trackball in the way. The ball is also very tight and does not spin as freely as the original wireless trackball. It almost feels "gritty". The battery life is superb with a single NiMH AA, although the original trackball had excellent battery life as well. If you suffer from CTS, get a wireless trackball. There is far less clutter and it just plain works.Overall: 4/5 stars! 

Question: How is the button?
Answer: The trackball now has additional " back / forward " buttons although they are located in an odd location that requires you to use your index fingers