A whole lot of fun!
The campaign is very good, by the end of it (which wasn't too long, but it took me a few days to beat) I was surprised at how attached I had become to some of the computer characters. The campaign follows the end of WW2 in both the Russia/Germany and America/Japan fronts and every few missions you switch between which story you are following, but this isn't really bad. My first solo run through was on the 2nd hardest setting (out of 5 I believe) which definitely added challenge and there were a few parts where I died many times in the same place, but I blame the difficulty more than the game. The game also captured the feel of war very well. You are occasionally ordered to do something by name, but usually you just feel like a soldier that is part of a bigger army.
Even though the campaign was good, people really want COD for the multiplayer and World at War does not disappoint. The game supports 4 player splitscreen and online co-op campaign which is fun and you can play with a point system so there is some competition between players.
The competitive multiplayer is also a lot of fun. There are several good levels and the perk for playing is that as you gain levels/ranks you unlock more weapons and perks. Also as you use weapons you can unlock iems to add to the weapon through certain challenges (e.g. if you get 25 kills with an MP40 you unlock a silencer). Also as you get kill streaks you get one time use perks (at 3 kills you get a recon plane that shows where enemies are on your map, at 5 kills you get an artillery strike, and at 7 kills you unleash dogs that run around and kill enemies in 2 hits if they are not killed and they respawn for a minute). I have found these to be very balanced and give you something to strive for as you run through the hectic battlefields.
Possibly the most fun part of this game is Nazi Zombies. In this you are game you and friends (splitscreen or online) trie to defend a house from waves of zombies (Similar to the house battle in Resident Evil 4, except COD style, co-op, and longer). You can unlock new rooms that hold new weapons to buy, but they also open new windows for zombies to enter. It starts off a little slow but after a few waves you are trying to balance reloading, boarding up windows, reviving allies, and shooting zombies.
This game is also somewhat gory. It isn't quite like Gears of War, but bullets cause spurts of blood and explosives take off limbs very often (especially in multiplayer)
However not everything in this game is great, there is some problem with glitching online such as getting into an area where zombies can't attack you and the infinitely more annoying glitch that allows you to run underneath the map and you can effortlessly aim upwards and shoot your enemies. It's also a little disappointing that if you look down you don't have feet, but that's a pretty minor thing.
Overall this game is a lot of fun and I recommend it to any fan of FPS or WW2 shooters.