Tow bar mounted racks are very convenient, and keep the crap falling onto the roof of your car, but they make a big dent into your fuel consumption, and the drag is very noticeable if you have more than one bike on it; in my case the fuel consumption difference with a single bike on it is about -9mpg, almost 20%.
Also, as someone else noted, the Thule racks offer zero security, both when it comes to attaching the bike to rack and the rack to the car (but that is no different from roof mounted rack).