I have used the Logitech wireless trackman for some time now, running a mouse with it at the same time so that I have a choice of the trackball or a mouse.
On the plus side, it is convenient to be able to leave your wrist/hand in one position at times and just use your thumb for selection. The Left & Right click buttons are easy to use, and the scroll wheel is also easy to use. All of this can be handy for a change if you are doing a lot of mouse work. In addition you can move the wireless device around so a change in wrist angle is possible every so often.
Battery life using a rechargable battery is several months, bearing in mind I use both at the same time.
On the negative side, I do not feel I could use this trackball as a permanent addition without a mouse. It is good when run together with a mouse so you have the option of using either one. During long periods I do not find the thumb very convenient for accuracy or comfort. My hand at the 'thumb joint' would get very uncomfortable after continuous use. My thumb is also not as fast selecting various points compared to a regular optical mouse.
Summary: Worth having as an auxilary device, but not by itself. (You can run both a mouse and trackball at the same time by plugging the trackball into a USB port, and an optical mouse in a PS2 port. I have done this for several years with no conflicts or side effects.)