It worked really well for me when I was younger. The stomach exercises are a bit tough on the back though, and too much for me now. It's very, very dull though, but once you know the exercises you can always spread them out through the day as you don't get sweaty.
It's muscle-toning rather than fat-burning so you won't loose weight but you'll find that your posture improves and you look a bit thinner because your muscles hold you in better.
My old gp practice nurse reckoned it was all the callanetics I did in the 80s that gave me such a good pelvic floor. Have just had dc3 (well 6 months ago) and pf still not too bad considering what its been through.
It is very easy gentle exercise that works on core strength and deep muscle tone, as fc said it was a forerunner of pilates.
Have to admit it is really boring though, tried it again a few years ago and found I hadn't got the patience for it anymore. If you found pilates boring, you'll probably feel the same about callanetics.
here ya go, the videos are all on you tube. I remember doing the quick callanetics when i got home from work 3 nights every week - yes that was years before kids and other stuff that took up the time - I was toned tho