Hi,
I carry my bike around a lot; although for couple of reasons I put it in the back with the seats folded down which is not useful to some.
Various friends have all manner of racks but the ultimate solution for outside the car, although pricy, is a tow bar mounted Thule rack. Expensive though to add the tow bar and the electrics for light board etc. They also fold down easily so you can still access the boot etc.
After that there are various racks that mount onto the hatch itself and strap down - some are better than others but all have the disadvantage of preventing access to boot when bikes on, and they put all the weight on the hatch. Some can move about a bit as well and mark the paint etc. As for roof bars; the ones where you remove the front wheel are a little easier to lift and secure but you do still have to cart the front wheels around in the boot.
So all depends on what suits you and your circumstances really.
