They work fine - I have one - but you need to secure them to the floor and the supplied tyre bars are a bit cheap and flimsy. If you get a really good tyre bar, the cost goes up considerably.
You also need an air compressor to fast inflate and seal the tyre to the rim, otherwise you're taking loose wheels to the nearest garage airline.
You'll also have to balance the wheels, so that's more kit and cost.
Unless you're getting through loads of tyres each year, I question whether they're worth the outlay over taking loose wheels to a fair-priced tyre fitter.
