I love pilates, but I've never used one of those balls. I don't go to classes, but use tapes/dvds at home. It's not the best workout ever, but it's generally a great form of stress relief for me and can actually be fairly relaxing. And you definitely feel stronger and more toned after doing it for a while!
I love the Method tapes, especially the ones with Jennifer Kries. Romana's Pilates series also has good instruction.