When it comes down to it, Call of Duty: World at War is essentially Call of Duty 4, only set in World War II. If you liked COD4, there's a pretty good chance you'll like World at War. If you didn't ... well, this game probably isn't for you. It's not quite as good as its predecessor - vehicle combat isn't as smoothly integrated into the game as it might have been and feels more like a distraction than anything else and the missions are usually on the shorter side, to name a couple of letdowns - but it's a shining example of how to do World War II well even when most of us are already sick of it.
Bottom Line: It's COD4 set in World War II. Awesome, cinematic visuals with a great sense of context and smooth tried-and-true gameplay, though short levels, tacked-on vehicular play and a buggy PC version keep it from being as good as its predecessor. Still, WW2 games don't come much better than this.
Recommendation: If you liked COD4, pick it up. If you didn't, avoid it. If you really, really love killing Nazis, you could do a whole lot worse than World at War.