I haven't used a trackball in years, but I do remember really lovin it when I had one. Something about trackballs always felt more fun to use. There's something satisfying about spinning a ball and stopping your pointer on a dime right over the button you want to click.
That said, they're not practical for me. Bigpook mentioned something about gamers. I used to be a hardcore FPS gamer and I don't know a single FPS player who's any good that uses a trackball. I also have to do graphic design fairly regularly as a Web developer. I've never tried to do design using a trackball, but I suspect it'd be challenging.
I used to be a physical therapist so I'm an ergonomics freak and trackballs really appeal to that side of my interests, but with the degree of precision available in today's mice and the availability of mice that don't hurt my hand with extended use, the only reason I have left to use a trackball is because it's fun. Before we had all these laser mice that are so damn precise even when accelerated, I could get a productivity boost from trackballs because I could zip my pointers around so quickly.
This is mainly because they're not competitive with modern gaming mice. There isn't a produced gaming trackball that has the functionality, or more importantly dpi of mice out, with the exception of my ugly ass prototype.
I use my hacked trackball which was built from a microsoft sidewinder to own in fps's consistently. I have a min of 2 to 1 kill ratio in almost every game I play.
I primarily play war rock right now, even with the hackers I own there.
The main deal with getting a trackball is that it should be index finger based, you don't have as much control with a thumb based model. Right now the only one that's at all useable in fps's that was a commercial trackball is the tbe.
As far as design goes, trackballs are still used heavily with 3d cad and 3d animation designers.
Normally with trackballs you have a great deal more of precision, but you typically sacrifice smooth linear motion, as others have said. But again this is typically a problem of low dpi of sensors in older trackballs. If you try to like draw a line with a tbe in photoshop, you'll notice it's a very jagged line, this is because the sensor is only about 800dpi, where most modern gaming mice are upwards of 2k. This means that in modern games it's difficult to use even a tbe because your precision doesn't make up for your lack of turn rate relative to the amount that you turn the ball.
This is solved with my hacked trackball because I can switch on the fly the amount I want to turn, either for precision if I'm a sniper, or fast if I'm playing an assault class. So I can turn on a dime, and keep turning if I need to.
This has an advantage over mice even because I never reach a limit in close combat, and can keep turning and dancing as warrock allows.
You also have to get used to the setting you have in the mice settings. I use trackballs with no acceleration and the sample rate turned maximum. I think this allows for the most control.