+1 to what
Damian wrote. My thoughts exactly.
It takes some getting used to, but it is wonderful.
For a trackball mouse, I like to put the acceleration way up and normal speed down. A quick spin with the thumb can cover a lot of ground across the screen, while small movements can still be used to accurately position things in Photoshop and other design tools (or just hitting buttons on screen for that matter, hehe).
You should spend some time getting those settings just right. Then stick to it and learn how to use it.
Unlike changing keyboard layouts (e.g. Dvorak), learning how to use a trackball will not really harm your ability to use a normal mouse.
For gaming though, you will probably be maintaining your "edge" in one or the other. For me, gaming with a trackball works well (FPS and strategy games), but I don't know if it would work well enough for really high-level competitive gaming. For recreational use it works fine.
And ergonomically speaking, it's a really comfortable way to interact with your computer.
Logitech M570
http://www.logitech.com/en-us/product/wireless-trackball-m570