Nirth: I forgot to recommend this one:
M570 Wireless Trackball. Not exactly suited for action games (with training perhaps that is still possible though) but you only use your thumb, no wrist straining.
I have that, but rarely use it as I found out that I can easily use a (wireless) mouse from the couch too, so I don't really need the trackball.
It is a pretty good trackball, but I just couldn't get used to it for high-speed FPS games and such. It is better than a trackpad (or a gamepad, for heaven's sake!), but not as good as a proper mouse. So, for me it is only for those rare cases that I really can't use a mouse comfortably, e.g. no even space at all to use a mouse.
What I like in it as a trackball is that it mimics a mouse in design, considering the mouse buttons, wheel etc. Most trackballs don't have similar design (but you move the trackpall with your "mouse button fingers), so to me the M570 feels closer to using a proper mouse, than other trackballs.