Hello. My name is Hawkgirl. I am a trackball addict.
In my experience, it takes about 2 weeks for full mouse-to-trackball switch or vice versa. You can get relatively comfortable in a few hours, but the precision dexterity takes a while. Once your muscles learn what to do, though, you'll be fine. You stop having to *think* about what you're doing in a pretty short amount of time.
And for me, it's worth it. Better control, less movement of my hand/wrist, and comfort.
There are different species of trackball-- the kind you move with your palm, the kind you move with your second and third fingers, and the kind you move with your thumb. I have a thumb trackball; the position of the hand is similar to using a mouse (e.g. you use the same fingers to click the buttons or scroll as you would a mouse), and it only took about a day to adjust. Besides offerin the most control (as my thumb dexterity is far greater than my middle finger, or for that matter my arm), I also like it because it requires the least wrist movement, an oft-touted advantage of trackballs (vis-a-vis carpal tunnel syndrome). I don't like the big palm ones, and in fact it's been so long since I've seen anyone use one it's possible they're not even manufactured any more.