Yes there is no dedicated server, and yes, you have to have an internet connection and steam, but it's still great fun
That said, this still has the same gameplay from modern warfare with some minor tweaks. I've only managed to play the single player so far and get past the first 2 levels on veteran. The training camp part was great and you know Infinity Ward, they always pack a lot of action into the story and so far they haven't disappointed. One thing I am disappointed though is the fact that you can't peak around the corners anymore. Graphics look great and most systems should be able to run it. I am using a Dell XPS m1710 with a 2.33ghz t7600 dual core and a 512mb 7950gtx, running windows 7 32 bit with 4 gigs of ram (3.25 useable). Getting decent framerates with mostly everything on, 4x AA at 1280 x 800. So far this game looks to be a winner.
Update: Just finished the game on veteran. Wow, what a great story mode, just so many times where you will be like "HOLY !!!!", gave the multiplayer mode a try, connection was very good and fast matchmaking of relatively similar rankings, however, players will be very disappointed by the game's support of a maximum 9 vs 9 multiplayer. I remember what made COD4:MW so much fun was the amount of players in a map so that wherever you went, the action was always around. It doesn't feel like that in MW2, as the most you will play against is 9. It's still a lot of fun to play though, and I think the bashing a lot of people are giving are just simply angry fanboys who can't enjoy this game that, while it's still a lot like the original Modern Warfare, is still a lot of fun.
Update:1/20/10
So now that I have finished the campaign, I have been playing mostly multiplayer. One thing that I should add is that there are noticeable hackers online. You will most likely run into them when you do play online, not very often, but most likely you will see them. Whether this is an issue with the VAC system implemented by steam or just IW's lazy programming and support can be argued, but most of the time, if you leave and join another game, you should be good to go. There was a recent period where people hacked into the console and made it so that one kill would get you a high number of experience (something like 6.5e) so these hacked servers would allow any players who joined to level up VERY fast. But the new update has put a stop to this (for now) and also fixed a texture hack where players could easily spot enemies with a texture hack. With that said, multi-player is still a blast, and the co-op is also there for people to team up with and play. The only real complain I have is the lack of a dedicated server, where an admin can just ban a hacker, and online rooms, where you can't type messages, but if you do have a mic, you can talk to others before the game starts. I originally rated this game a 9.5 but after playing for a couple of months, I have decided to lower the score to an 8.5, because I don't think that this game is "Superb". I think it's a great game, but it could definitely improve.