Ok, so I've spent the last 3hrs playing an even amount of both SP and MP.
Single Player: No spoilers, I've played the first few levels. I'm as far as the Brazil mission they showed online. One major "I didn't see that coming" already. From the review's I've read though, if you're only planning to pick this up for SP, then wait for the bargain bin, because it's short. Average play through on normal for a CoD4 vet has been 4-5hrs. 7-8hrs if you play on Veteran.
Multiplayer: This is why you buy the game. As tight as ever. I've played a half-dozen rounds and I'm already hooked. Matchmaking hasn't been bad at all. Very easy to jump in and out of games. I can sum up the level design in two words: Vertical Gameplay. I have to go back to UT2K4 to find a game that had so many options to traverse a map. So many cubby holes and back doors it's sick. The larger map size and smaller teams was working surprisingly well. Advancement is a joke so far. They reward you for absolutely everything. I'm Lvl 7 already after only an hour. Instant gratification at it's best. Not complaining again, I'm a tool, I love it
Complaints: These are minor gripes so far. The retail packing should have been stamped "Steam Offline Installer." The very first thing it asks is for your CD key and then informs you it's permanently associated with your Steam account. If you already have Steam installed, do yourself a favor and start it before you start the install.
The instruction book is 9 pages, 4 of which are legal type, and one is a blank "Notes" page. Why the pretense of a real manual, just give me a sticker with my CD key and save that extra 2 cents it costs you to print the manual. Two DVD's 11GB and it doesn't give you the option to install to an alternate directory. Lucky I had 11GB free on my C: drive.
Overall: I think most of you would really like the game. I haven't even tried Spec Ops yet, but that should be great fun for when only one or two of us is on. With the party system and TS, we can stay together as a group as we pub, or if enough of us our online, start a private match. I'll add updates as I play more.