Yay! Those things are great.
(<--I looked for a stability ball smiley, but, well, smileys ARE balls, so I just picked a bouncy one.)
I don't feel as guilty spending money on exercise equipment (or having it take up valuable apartment space) because it's an investment in my health, which I'm trying to make one of my top priorities.
I'll admit that I don't take full advantage of my ball... usually use it once or twice a week, for abs or lower back. But it's so nice, you can do a million things on it... you can even use it in place of a bench for some stuff, like chest flies.