My partner is an electrician and he fits underfloor heating sometimes. I think it is fairly easy to fit, although it is advisable to get it done by an electrician, cos with the new laws you'll need a part P certificate. Usually people tend to have it in bathrooms, or smaller areas such as flats. Although I have seen it put into larger rooms.
I don't think its any more or less expensive than say storage heaters to run. Put it this way, nobody's ever complained to us of enormous bills or anything