Exercising my curiosity

Exercise balls make me laugh at how silly they look. But at the same time, I must have one.

All the cool (and fit) kids seem to be working out with them these days. And several new magazines I've fallen in love with illustrate a zillion different workouts you can do with one of these simple, plastic balls. It's truly amazing.

So, I need help. A simple search for one of these balls on Amazon.com turned up 680 results. Who new there could be so many sizes, colors, strengths and brands of these things?

Have any of you workout out with one? Is there anything I should know about all the different sizes (55, 65, 75 cm, etc.)?

I realize I'm a little late on the whole exercise ball revolution, but I guess it's better late than never, eh?