We have a £4.99 Tesco ball that's stayed inflated for two years - can't be bad.
Size is probably more the issue - they come in different sizes. If you lie on your back, with your calves on the ball and your butt close against the ball, you want the angles at your pelvis and knee to be as close to 90 degrees as possible.
Some exercises on balls are damn hard to get the hang of. If you want a wet-yourself-laughing evening, have a couple or few drinks with some friends, then get the exercise book out and get everyone having a turn.
Annie was our resident expert on balls...