Here's my opinion on the whole thing... think of it as a review if you will.
Singleplayer
As with the other games in the CoD franchise, MW2 has a great cinematic/engaging singleplayer experience. While a bit on the short side (I got through it in 6.5 hours on Veteran), it's one of those hard-to-forget thrill rides that - if you like this sort of thing - you'll not regret having played through.
Also, I suggest choosing "no" on the content censor box that will come up when you first run the game. Granted, the level it removes/skips is a bit like Frankie Boyle's humour - one of those things you might feel slightly guilty enjoying - but the level is quite well done with a slightly unexpected ending. It's sort of like a deleted scene on a movie DVD. There's not much story to be found in that particular level, but it does give you that little extra insight as to what's going on.
Multiplayer
Okay, this is why you're reading this far down.
As many people, places and petitions have said thousands of times in the past month, the MW2 MP is missing a lot of obvious things that CoD4 possesses. If your enjoyment stems mainly from the competitive aspect of FPS games, you'll find the MP side of the game sorely lacking, though that'd mostly be from the lack of team-switching/balancing and a level playing field in the form of ProMod. However, if you're open to playing a game for the sake of doing just that, there's some fun to be had.
As for the lag issue, I haven't really noticed much difference between playing MW2 and CoD4. Servers on CoD4 sometimes have issues, just as you might get lumbered with a crappy host on MW2. The only real thing that lags more than is desirable (in an optimal situation when all goes well) would be the throwing of grenades; there's quite a noticeable delay between the animation ending and the actual grenade beginning to fly.