Interesting there is a huge debate among bonsai owners whether cat litter is suitable for planting bonsai trees in also. My main concern with things like Tesco lightweight and Sophisticat pink (the two main ones used by bonsai growers) is that they contain perfumes etc and whilst these may not have any adverse affects I wouldn't want them a) near my trees or b) in my fish tank.
That said, and as you can see from the above replies, plenty of people do use it and are very happy with it (as they are in the world of bonsai). Just make sure you get the right one otherwise it will clump horribly!
There are plenty of unperfumed molar clay (which is all this stuff is afterall). The stuff garages use to soak up oil spills with often works and various bonsai nurseries sell unperfuemed stuff as well.
Or, you could go for Akadama which is a pure natural Japanese clay - again your local friendly bonsai nursery is the way to go. Its more expensive but top quality stuff.
after a few years of testing various 'specialist' substrates I now only use the cat litter, it's cheap and is as good if not better than some 'specialist' substrates, the only fault with it imo is the colour but then once the plants have grown you don't see much of it anyway