Its like Diablo, but with guns!
The missions are very good, with killing and fetching quests etc. But its all done very well and its very fast paced and filled with dry humor (the best kind).
If youre playing for the first time all the classes are very viable but I have felt that 2 stand out among them and are clearly the best. But if you are playing with 1 or 2 other people it will be a very balanced experience with each member contributing something a little different.
The real draw to this game is the items which include all types of guns, shields, grenades and powerups; all of which are named in a similar fashion to other RPG's. They have various color text which represents rarity and stats such as damage, fire rate, etc. The worlds are full of chests containing these items and you can also get them from bosses, and mobs. It really keeps you and your friends playing and when one of you sees an orange item drop everyone immediately runs to grab it (then you pause and argue who should get it before continuing to play).
Very fun, very addictive game that draws you in and never really gives you a reason to leave. As a bonus all 3 expansions are very well done as well. The zombie one is by far the best part of the game, and the underdome is very intense. Generals Knoxx is a much different turn for the game and is far more difficult so it takes much getting used to after playing the main game and first 2 expansions for so long.
Overall get it if you plan to play on LAN or possibly online (tho I havent tried that yet). Single player is good, but only to help you get ahead and find new gear before your next LAN session.