Don't listen to these haters on COD4 for the DS, they're probably bitter it isn't some huge expansive world of the PC title and lacks the online, many FPS people get blinders over that.
COD4 while it won't take more than a few hours to finish has multiple challenge levels and the depth that the DS allows should not be ignored. YOu can get it I think $15 used/20 new these days (or something like that if you're buying) so I'd say get it. It's not halfassed, it's not a 'beat the mission bit' either anymore than the full scale game as it was done in bite sized chunks too.
I do agree though that World at War is far more advanced, smarter AI, and has some more things in general to do, but what do you expect for a 2nd generation CoD title...going backwards?