My previous trackman was great which you operated the trackball with your thumb. It was accurate, very comfortable, eliminated the strain I was having with a normal mouse and was very natural to use coming from a normal mouse. When the left button started playing up I changed it for one of these. Having the trackball operated by the middle fingers and the left button with my thumb felt incredibly unnatural and i found it very stressful to use. Every mouse I've had had the buttons operated by the fingers and I couldn't get comfortable with this design.
Other people seem to like it but if you're used to having the trackball operated by your thumb be careful, you may be better off buying the more expensive wireless version which does have this configuration.