Annoyed but it doesn't break the game completely. The lag is barely NOTICABLE (not that it isn't there but the game handles it quite well).
It can get annoying when the system breaks but its still a really good game. Especially if you have a few friends and you just host a private game with your own settings etc.
Also, MW2 doesn't have them but the original Modern Warfare does, which is why it is the better game (also, it's more realistic and feels a lot more solid) Call of Duty 4 is pretty much the best of the bunch and it's also the oldest.