haha just when I was going to put a vote in for the 750Li, ... looks like you've already made your decision! (which, btw, is a great one)
FWIW, I've driven both the S550 and the 750Li, and I'll have to say for driving pleasure the 750Li wins by a longshot. Don't get me wrong, the S class is nice, but, as Phong (BreytonX5) mentioned, the new S gets a lot of its styling cues from the E65/66. The 7 series being cheaper than the S, with included full maintenance, and lease rates that kill the S-class (if you're leasing), you can't go wrong with the 7
In regards to pricing, I know the stealer by me will sell 750Li's for 1,000 over tissue (invoice)
Good luck! By the way, what option packages are you considering? You're probably going to get luxury seating, premium sound, 19in rims, convenience package, comfort access, and the sunshades? Don't forget the rear entertainment package (pricey at 2,200...although it's great to have a 6-DVD changer for the rear seat, and control of the audio, it's an exact copy of the front seat i-drive controls...if you look closely, you'll notice the screen of the rear entertainment package looks much like the navigation screens in our E53). Rear comfort seats are great as well (a 3,500 option). That adds power rear seats, and ventilation.
The 760Li that wagner mentioned is out of this world! If you do go for a 760, a must have is that rear coolbox option with the independent rear ventilation controls!
If you can't already tell, I'm a big fan of the E65/66. Being an E38 owner, I hated the current body style 7 when it came out. After driving it, and seeing it more, I began liking it. After the facelift for MY06, I like it even more, and wouldn't mind owning one! They're great cars!!
Good luck!