This game is just awesome, and the amazing expansions keep me coming back for more!
A lot of people have already described at 'Diablo with guns', and if you played Diablo and this game, you will see what they mean almost instantly. The mixture between first person shooter and RPG really works out here, and the artsy graphics and design, combined with the always present humor in the characters and situations is just plain fun. It's a game definitely worth playing if you like RPGs and first person shooters.
The multiplayer is where this game really shines. You can play with 2 people split screen, or with up to 4 online, and the more people playing the harder the enemies will be to kill and the better the loot that will appear. And man is there a lot of loot.. you gotta believe them when they say they have a bazillion guns in this game.. because they really really do. There are just so many variations of weapons and shields and other items that no matter how good your arsenal is, you will ALWAYS be on the lookout for better versions of your weapons, or brand new awesome weapons, and with more players you will get a better chance of achieving that pretty quickly.
The weapons themselves are organized in different rarities, as are the other items like shields and stuff. They are organized like this through colors, from least rare to most: white-green-blue-purple-yellow (familiar?). There are also even more little details on that, the last rarity has different degrees: lighter yellow weapons are the least rare of the rare weapons (wow), and more orange colors mean you really hit the jackpot. With the last expansion pack to come out (general knoxx's secret armory), they added another level of rarity, known as the pearlescent weapons. These are the rarest of them all, but sadly (if you don't have the DLC) can only be found in that expansion pack.
About the other items i mentioned, there are a few types of them. The shield, which works a lot like the shields do in Halo, giving you essentially an extra life bar that goes down before your actual health starts being depleted. They recharge after being depleted, and their recharge speeds and rates are variant, as well as their strengths and other special features such as resistance to a certain type of damage. Another kind of item is the Grenade mods, which make your grenade deal more damage, different forms and different types of damage. There is also Class mods (there are 4 classes for players to choose from in the beginning of the game: hunter, berserker, siren or soldier), which give a specific class of player different skills and conditions.
The classes of players are overall pretty balanced, but for some situations there are classes that are better than the others. The hunter class is more of a sniper style, concentrating on sniper rifles and pistols. The berserker is a more melee and explosions kind of class, with great advantages using rocket launchers and shotguns. The hunter is more of a combat rifle class, using mainly those as well as SMGs. The siren is more of a elemental character, being great for dealing different types of damage and using mainly SMGs. Of course, the player is not limited to these weapons per class. You can be a Hunter (like me for example) and still use combat rifles and rocket launchers with no problem at all. There are no real limitations, but if you use the guns meant for your class your performance will usually be at an advantage.
There are also different powers for each class. The hunter uses his pet bird of prey (Bloodwing) to attack enemies, the berserker goes berserk (appropriate) and punches the heck out of anything that moves, the soldier deploys a turret that helps in the fight and provides cover, and the siren can 'phasewalk', becoming invisible to enemies and getting a boosted melee attack to kill enemies for a short time. These powers, along with a whole bunch of special skills are upgraded every level (though powers are acquired at level 5), and here is where one of the biggest RPG elements of the game comes in. You can distribute these points in any way you like, getting one every level since level 5. If you change your mind about the distribution, the game generously offers a system where you can redistribute these points in any way you like for a fee, which is just plain awesome.
I'm not going too much more into the game now, because this review is already too lengthy (as my reviews usually are), but if you really haven't tried this great game yet, do yourself a favor and check it out. If you are not sure give it a rent! I guarantee you will be addicted if you enjoy FPSs and lots of loot (who doesn't right?). Personally this is an all time favorite for me