I can't imagine playong MMO or FPS games with a Track Ball - that's hard 
I've used nothing but a trackball for years (specifically, the
Kensington Orbit), and I don't find it at all difficult to game with one.
I'm not a hardcore gamer by any means, but I've played many, many an hour of EQ, TF2 and many other games with no problems. Before I got my trackball I played EQ with a mouse, and had lots of wrist discomfort after long sessions, but it was much better once I switched to trackball.
That being said, I'm interested in this topic because I also wouldn't mind having a few extra buttons but there don't seem to be many good options out there for trackballs.There's the Kensington Expert Mouse and Slimblade, both of which have 4 buttons. There's also
this that I came across, which has 6 buttons, but it's unclear to me how configurable/assignable they are. It's definitely not made for gaming and I don't see any info on dpi, etc., so I don't know how suitable it would be (plus, it's huge and sort of ugly, but those may or may not be a big deal).
PigLick