I swear this isn't a super obvious hint to you. (You know who you are)
Borderlands is a gritty blend of RPG and FPS four-player co-op game. The co-op is essential - find some friends and make them play with you, or else don't bother playing. You don't get any AI to help you, so you need those other players. Oh, and make sure you name your characters with ridiculous names. Any game is made better when your character is named "Fuck Machine" or "Dance Baby."
The game is set on another planet, filled with horrible beasties and baddies and rumours of a vault filled with alien technology, money, fame and women. (Don't ask how the women are still alive after being in a vault for so long). The plot is tenuous and thin, but really... who needs plot when you have things to kill? And they've added several DLC packs to the game that add plot of their own - zombies and a robot revolution, notably.
You play one of four treasure hunters, each character representing a "class." The classes are best described as tank (MEATY), sniper (LONG RANGE. Also he has a falcon.), elemental (PHASEWALKING and ELEMENTAL SMGS) and soldier (GUN. SUPPORT. Also, TURRET).
It is the perfect blend of grit, gore, sick humour, and guns. As the trailer said, over a bazillion of them. Since there's alien tech lying about, you get to play with more than ordinary bullets. Elemental weapons are the trademark of the game - you have a chance to trigger an elemental effect that will completely destroy an enemy in the most horrible ways imaginable. You haven't seen grit until you've seen someone's skin be fried off their skull and their eyes pop out because you shot them with an electric gun. Or watched as corrosive acids melt them away into nothingness and they run around screaming and flailing. Also, there are midgets. They do the best flailing.
The RPG aspect is simple, but effective: as you get XP and level up, you can put skill points into crazy skills that make you amazing. Turrets that can spawn ammo or regen health, guns that can bypass enemy shields, explosive phasewalking... it's all there and it's all good.
This isn't really a review because in no way am I being balanced and fair, but I honestly can't think of many criticisms. The plot is somewhat thin and the mapping system is a little difficult to use, and the menu UI can be a little funky some times, but other than that I have enjoyed every minute I've played this game. We're actually going through our second runthrough at the moment, and we've only touched half the DLC. The game is a great size, physically and time-wise, and once you beat it the first time you can start a second or third playthrough where the enemies are harder and you're more badass than ever.
So, you should play it. Because I told you to.
(If you can, buy it off Steam on sale - Steam will manage updating the game for you (retail version you have to manually patch), and the retail box version requires a disc to play! )