Neat little package. This game may not be the genre breaker that it was marketed as, but what it does, it does right.
First off, gameplay. Borderlands has solid controls, nothing like the almost-clunky Fallout3 in terms of FPS. Guns have a solid feel to them, animations and sound effects are smooth when shooting and reloading. Enemy AI could use a little work, but most of the time I was fighting off several enemies at once. They will flank and use basic group tactics, but if you find a spot not so easy to get to, you can easily pick enemies off one by one (in your level range). I liked that enemies didn't necessarily level with you, there is always a chance you can round a corner and find a beast 3-5 levels your senior. Run.
Vehicles will take some getting used to as far as handling goes, but mowing down bandits with rockets and your front bumper is very satisfying. There are only 2 seats in your basic vehicle, which can be a drag for co-op with more than 2 people.
All zones are basic instances, monsters and barf piles will repopulate. Some may find this annoying, but it makes for good fun with any MMO/Diablo2 fan. Each zone is unique enough to be interesting, there is some good level design here.
I find the art direction to be spot on, everything has a Mad Max-meets-Starship Troopers-comic book feel. The graphics seem to push the Unreal engine in a pleasant direction. There is a bit of aliasing which can be force corrected, if you're willing. Enemies are well rendered and various. The gore level is not over the top, but definitely not understated. A very classy take on post apocalyptia.
As people have said, playing this with friends is where the fun is. That is not to say playing solo is boring, far from it in fact. But the frantic, giddy chaos that is 4 player co-op is something not to be missed.
The RPG elements are somewhat muted compared to the fast paced action. There are levels, weapon proficiencies and class specific skill trees. This is your basic Diablo style levelling system, minus physical stats like strength, dexterity and so on. While this system is integrated very well into the game, it could be a bit more in depth.
The questing system is simple, yet well executed. Having found no bugs in the game thus far (about 15 hours in) I am pleased overall. There are perhaps a few too many "fetch X" or "kill X number _" quests, but any RPG fan will know this is par for the course.
Overall, I have been having a blast. This world of Pandora is colorful, ruthless, funny, full of weapons and rather large.
I plan on spending many hours with this game.