Take this little co-op anecdote for example:

So there we were, myself as a level 20 Siren, Petey as a level 21 Soldier, getting ready to take on Skagzilla, a huge, heavily armored version of Pandora's resident dog like beastie. "Now get ready," Petey says, "'cause once I put this bait down, he's coming outta that cave." He pointed to a very large cave, no doubt home to a very large beast. Now, I'm a Siren so my proficiencies lie with SMG's but an SMG against a foe this large seemed difficult so I equipped my scoped rocket launcher. Petey puts the bait down, there's a terrible roar from the cave and Skagzilla emerges. Petey throws down a turret which begins chewing up Skagzilla while at the same time healing anyone who comes close to it. My class mod gave us extra shield power and Petey's makes it so that we regenerate ammo meaning that I could keep firing a steady stream of rockets at the beast and never run out while Petey could take advantage of his shotgun proficiency and get up close without worrying about his shield. We continue to whittle down the beast's health until I decide that I want in on this up close action as well. I activate my phasewalk skill doing fire damage as I go invisible, dance around Skagzille, doing electrical damage as I do so, then hit him with a corrosive melee attack to be come visible again doing even more fire damage as I come out of phasewalk. Two blasts from Petey's shotgun later and we've got a new Skag skull for the mantle. I never even ran out of ammo.

Having a well equipped and well balanced team makes the game so much more enjoyable, I'm not sure why you'd want to play it alone. The character classes, all seemingly equal at the lower levels, really start to shine as you bring them up in level and start specializing their skills. Each class has a skill tree to put points in with three distinct paths. One will buff up special skills while the others usually buff up weapon proficiencies. The system works fairly well allowing you to create a character class that follows what the developers had envisioned for the class, or make something unique like a revolver toting berserker. With class mods and elemental artifacts to apply to your special skills you can even further tailor the gameplay to your liking as well as make yourself an essential part of your team.

One of the biggest selling points of the game is its jillions and jillions of guns and while there may not be so many weapons in the game as to require fake numbers, there sure are a lot. Weapon drops get better as more people join the game which unfortunately highlights one of the game's biggest annoyances, namely the lack of a proper trading system.

Sure you can just drop a weapon on the ground and then someone else can pick it back up, however having the ability to just give a weapon to someone, or better yet, gift one over Live, would have been fantastic. Playing with the right group of folks becomes essential when going for loot as it's too easy for some random twerp to come in, bootleg all of the good guns and then skedaddle. The game sports a Duel mode where exchanged melee hits between players allows them to settle their grievances a la the old west however this mode isn't mandatory for resolving fights over who gets that awesome static sniper rifle so diplomacy is still your best bet for making everyone happy. Luckily, as you progress in the game, the higher level weapons become less and less effective for those that haven't built up that particular weapon's proficiency so playing with a balanced team means that a lot of these arguments will settle themselves but seeing how you can always sell a weapon for cash, it's always valuable if only to fund the next purchase.

That being said the guns are awesome. Whether it's a scoped revolver that does electrical damage, a three rocket rocket launcher, an alien rifle or a shotgun that sends out explosive ammo you'll find something that tickles your shooting fancy, something you can't bear to be without. Until the next drop that is.