I recently purchased the Paddy Hopkirk (Mont Blanc) Quickball cycle carrier after reading the reviews on Amazon and the BikeRadar.com review. This carrier requires a towbar fitted to your car and the carrier clamps to the towball.
I was a little unsure about how good this rack would be but I am very pleased. Fitting is very fast, and once fitted it is very secure. So far I have only used it to carry one bike, but believe it will be fine with two. I use a couple of extra bungee straps to secure it, and also use a trailer board as it obstructs the lights.
If you have a towbar and are looking for a relatively cheap way of carrying one or two bicycles then you should definitely consider this.