I'll have to second the "they're fascinating" answer. I love how even my small collection has a wide range of personalities - not as obvious and in-your-face as a dog, of course, but still obvious to their keepers.

I also enjoy watching my only live holdout hunt. Not for the "thrill of the hunt" or to watch the mouse get killed (I always feel terrible that I have to drop the poor thing in there), but to observe the most natural act this wild animal can have in my hands. It's really interesting to see how she sets herself up for the ambush, waits for just the right moment to strike, etc. Definitely can't wait till she'll take F/T though.

... Of course, you can't beat all the shiny colors either