I bought Black Ops on the recent steam sale during the free MP weekend they had. The MP is not all that bad (but still not as fun to me as TF2). Reminds me of CS in a way (MW did too). I had a fairly decent time in MP during that free weekend so I figured what the hell. The thing I do like about it is the server browser. I can't seem to find one in MW2 (which I got in a recent 50% off sale on steam) so joining a MP game there has been lame.
I played some DNF MP last night and while it was kind of interesting overall it was laggy and just didn't feel right. It's shocking for me to say this, but I have a lot more fun in CoD MP. I'm usually very critical of CoD due to the direction its SP has gone and MP is something I don't play that much.
Still, I rarely buy DLC and the few times I did it was on a sale where it was extremely cheap. For the most part though, paying for DLC is just lame. Especially at the prices it's usually set at.