Admittedly, I've barely touched the single-player mode, but I find the multiplayer to be a true gem. I've put a lot of hours into MW2 and BC2, and in comparison I definetly have more fun with AvP.
I admit that the race balance isn't perfect, but since they are so asymmetric it doesn't ruin the fun.
The biggest issue is the lack of population, partly due to fragmentation between dedicated/listen servers, ranked/unranked modes, etc. But most other good multiplayer PC games also have population issues.