In reply to LastBoyScout:
I've tried a few different car carrying setups over the last 10 years for my 2 adults + 2 kids bike car carrying requirements. So this comes from my hindsight learning....
Firstly, kids grow and so do their bikes. If you're happy to lift 4 bikes on and off the roof of your car, then the main advantage to having roof mounted racks is that you can leave them on (when not in use) if you're a frequent user (albeit with a slight fuel penalty). Downsides are all the heavy overhead lifting, having to remember to chant "bikes, bikes, bikes" when there's any chance of encountering a car park barrier and the impact on your fuel economy especially driving into a headwind. The Thule 598 style with windy ratchet clamp are really good and have put up with a lot of use and abuse from me.
However, as the kids have grown, it became a real chore lifting the bikes on and off the roof and my wife just isn't tall enough to reach the 2 racks in the middle. So, we invested (and blimey it felt like one) in a towbar and a Thule Velocompact with extra 4th bike carrier. It's a heavy beast that definitely needs some respect and technique in lifting on and off but.... once it's on, it's brilliant and so much easier to get bikes on and off. Probably still has an effect on fuel economy but doesn't feel quite as bad. Obviously a bit of care needed when reversing and parking. Overall, I wish I'd bought it 10 years ago.