I would play DoD:S rather then CSS any day. It is like TF2 but army style. The players communicate to each others. But, after you get the hang of it, do all the achievements blah blah blah. It gets really boring and becomes usless if you have lots of other games. The price tag is quite low. There is still a lot of people that play it. I don't see y you shouldn't get it.
It's still quite active. The community isn't really growing, but it won't be dead anytime soon either. It's a fantastically good game, and quite honestly the only FPS that I've ever played for years on end.
I have TF2, CSS, and even CoD4... I just never got into any of them as much as I get into DoD. Gaming bliss.
Like spetz I only really play DODs, I sometimes play C&C but only really when the latest one comes out (waiting on RA3). It's a great game and if you get addicted you'll never play anything else again. It has quite a dedicated community of mappers as well so when you get bored of the stock maps there's a limitless amount of maps out there.