At first I thought this was a terrible game. The unskippable intros, the claptrap that repeats his lines every 10 seconds, and the weak story and quests made me wonder why the critics thought this was such a good game.
After a few hours, I must say, it's addicting. The fights are exhilarating and the weapons are extremely diverse and thrilling.
There isn't much to it. Most of the quests aren't "kill x of y", but they largely are "collect all of x" or "kill this guy". At least there's something about having to kill enemies to get to the goal, rather than the goal being the actual killing that makes it feel less like grinding. The fact that even random enemies drop phat loot and that experience adds up quickly makes even the grind-y quests worthwhile and fulfilling.
And really, that's all there is. Like Diablo, the game is basically an excuse to kill. You kill things to get experience and guns which allow you to kill bigger things. As simple and stupid as it sounds, the killing part is so visceral and enjoyable that it doesn't really need anything else. At first the weapons seemed lame, but I quickly started getting amazing weapons, and just when I thought my weapon was overpowered, there was an even tougher enemy who dropped an even more amazing gun. I quickly became addicted to trying to find a bigger, better gun.
The game isn't without its problems. I could do without the mobs respawning as I'm leaving (especially when they respawn right next to me). Also, the characters often repeat their lines, especially the claptrap, who isn't nearly as funny as he thinks he is. In only a few hours, I also ran into several cases where scripted events didn't occur, forcing me to reload from my last checkpoint.
The cel shading thing is a gimmick if you ask me -- an excuse for critics to call it "artistic". The world doesn't seem all that unique -- it's a lot like Fallout 3. But the characters are interesting, the tongue-in-cheek attitude is fun, and none of it matters when you blast a guy's head off and leave his decapitated corpse to burn.
All in all, it's a fun game. It's not as amazing as the critics are making it out to be, but it's certainly worth playing. Despite the mediocre RPG elements, the overwhelmingly satisfying loot and visceral combat make it quite enjoyable. I hope to see more FPSs and RPGs adopt a similar procedural weapon generation system.