I've tried a couple and have come to the conclusion that they're not worth the hassle. Can't open the boot, scratch painted and plastic bits and you'll need to at least tie on another numberplate but maybe need lights as well depending on where the bikes sit. I now use Thule roof bars and bike racks. More expensive but much more secure and actually a hell of a lot cheaper than a respray. You can lock the bikes on as well, so you're not worried about them going missing when you stop for a coffee. The bars have been used for a hell of a lot more than just carrying bikes as well....really handy just to have em.
If you're going to carry bikes more than a couple of miles and more than once I recommend bars. Just my tuppence worth.
Oh, you can also get a rear rack that sits on the tow bar. These look like another alternative but they aint cheap either.