I've had a s-max for two and a half years, and its been great. i rarely use the rear seats, and yes, its the headroom that is the difference, although if I remember correctly the galaxy is 40mm longer, which maybe just the bumber? I use the Thule Ford supplied brackets and bike mounts, which pick up on the front forks – QR only, although I have had a maxle compatible version made. If you only had a front seat passenger you could certainly carry four bikes in this way, front wheels and seatposts removed. Its very flexible, as with only the centre seat of the centre row flat I can get my bike plus the two kids' bikes running down the middle, with some luggage either side.
One thing I would say is that with my large frame and a 140mm fork, the bars only just fit under the roof lining with the bike upright and bars to the rear, so longer forks would be an issue. This is where the galaxy would be better. That said, you could put it in bars fist, canted over, as it would probably be alright upright under the taller part of the roof above the centre row of seats.
As a car, it drives superbly, much more like a car than a MPV, and I have had very few issues with it at all. Its not as refined as a big German estate, it's a Ford after all, but I think its a great car.