I went with roof bars (cheapest square-section Thule) some years ago for an XM, now on a Zafira with other cars between those two.
It was easier to load onto an estate car than it is onto a midi-MPV, but still OK.
Why roof?
1. it uses the same lateral bars as the roofbox we already had.
2. The car get no wider with bikes on it.
3. Visibility to the rear is unchanged.
4. I hate webbing straps and the like - which some rear carriers depend on.
5. We can get into the boot/hatch with great ease.
6. I imagined a roof system would be easier to use on several cars - so far that's proved correct. Some tailgate systems seem to fit only a few models.
7. Some tailgate systems need a seprate number plate, which I just don't find elegant.
With things as they are, I can put three bikes on the roof, with a couple of surfboards (minus fins) strapped next to them. Or I can go with the roofbox and one bike. I could do box and 2 bikes, but I'm not enterprising enough...
There's also a boot for another couple of bikes sans wheels, although the cleated shoes, pumps, tools, helmets etc take up a remarkable amount of space with a family on board.
Bad things about the roof?
1. I'm short (5'8") and it can be comical putting the bikes up. I have to stand on the sill of the rear door. Getting them down it's 'best if we don't park near other cars'. In other words, I'm likely to end up falling backwards with bike over my head and a worried look on my face.
2. The clamps that go around the bike frame are fiddly to adjust - so I need to figure out which roof-thingy to use for which bike... super-narrow 531 steel road bike of crazy-fat Pace rc303 hardtail.... or something in between. That can be tiresome.
3. You must remember you have bikes on the roof. Those clangy horizontal bars on chains like the ones on the height restrictor at the McDonalds Drive-Through just outside Hereford will otherwise cause you to curse and blaspheme. Luckily, they are light alloy and do only superficial damage at low speed...
4. I dare say MPG suffers too... But life's too short to worry about that.
5. Security. If it's a single bike, I whip the wheels off and stick it in the boot. The boot is always favourite for me.
6. More expensive on ferries etc. with the extra height.
I drove to Paris a couple of years ago with teenage daughter to spend a w/e riding round the city. We took 2 old MTBs in the boot of a Ford Ka - with tons of room left over for luggage (seats folded). Great fun, nicer than Velib and the tiny little Ka swallowed the bikes in one bite.
So boot is favourite, followed by roof. In my not very organised mind, anyway.
