I only had online lag issues the first day or two the game was out, none at all since.
Matchmaking is a dream if you have a team of 5, it rarely takes more than 20 seconds in that case. If you have 4 people forget about it, it takes ages to fill that one gap before you can move on to finding opponents. No one plays alone I guess. I felt like the multi was worse than the originals that first night but now I like it a whole lot better. I guess if you play alone you would hate it but I always play with friends and it totally sucked trying to get 3-4 people into a match and then getting them on the right team between rounds.
I'm up to the start of Act 5, all on co-op with a good partner, and have no complaints about any part of the game. Fucking rocks.
If you want the achievement for having your hostage dying in your hands - stick him with a grenade and then pick him up as cover, it counts and it will not hurt you.