I've just bought a cheap as chips one £9.99 from Home Bargains....hand held one, apparently it will magicly clean my oven and bathroom from all the gunge that's accumulated because I'm a slutty housekeeper.
Loosens dried on dirt, e.g. High chairs, cots, tiles, dried on plaster, manky floors. Gets rid of smells on bedding, carpets, around toilets. Cleans glass on ovens. Makes light work of velux windows if you have squidgee attachment. I could go on and on.
Some people use theirs for removing creases from clothes, but I have always found it a bit of a gaff.
Major disadvantage is that you (or some other mug) has to actually switch them on and use them! Wonderful if you cba though. Can also use for ironing and steaming clothes, with the iron attachment. Wish I could afford a cleaner and then they could get everything spanking clean for me!