As the 550 comes with sport/comfort seats, I think they are pretty good. On my E60 I had comfort seats (sport) and they were decent. I need to take cholesterol medications which have caused damage my muscles...double edged sword. If I stop taking it, I feel better but I'll have a heart attack much sooner. If I take it, I can only take it at a low dose. The comfort vs. standard makes a huge difference. I set relatively close to the wheel, but wheel is kept in position more closely to the dash. I don't inflate the lumbar as I've found that to irritate my back more. I do not have much recline. The thigh support is great.
If you have a bad back or neck, the comfort seats really do help. It does take some time playing with it just to get the right spot. Once you do you'll really enjoy the seats. Some of the best in all of the car biz.
I would argue an old school 88 Volvo 740 had the best seats, but that is past history. I find on all cars for me the thigh support is lacking inless you get a seat that has adjustable bolsters. IMO the standard 5 seats aren't that comfortable.
A post above mentioned an MB GL. The GL shares the same seats as the last gen ML W164. I had one for just over a year. The seat length was long enough supporth my quads and knees, but the seats were hard and the car + seats transmitted road problems right through the seat into your body. If you have back/neck problems you'll understand.
I like the adaptive suspension on my 550 and the sport (comfort) seats. Extremely comfortable car.
But, I'm a little biased. Good luck guys.