Borderlands is a heck of a lot of fun. As a guy who has played a lot of post-apocalyptic games (yes, including Wasteland/Fallout -> Fallout 3), I have to admit that Borderlands hits that nice sweet spot between pure FPS and pure RPG. In Borderlands, you pick one of four characters and get to go for a romp in a fun quasi-post apocalyptic wasteland that is part Mad Max and part Pitch Black. Basically, you are on a different planet that is a rough and tumble outback world filled with nasty aliens, rogue humans, and lots of gunfights. It's a good time.
Borderlands is not as much an RPG as Fallout 3, for example -- your character is more of a Diablo style with skill trees rather than fully customizable. Quests are also less elaborate, and a bit more on rails with areas unlocking for exploration only as you advance the plot. Borderlands also reminds one of Mass Effect for its combat heavy emphasis and shield technology, but let's get down to some specifics:
1) Guns, guns, guns. I knew Borderlands was hyping its "17 trillion guns" bit, but didn't realize quite how awesome this would be until I really got into it. It really is amazing how varied the weaponry is. With just a 20th level character, I've already found sniper rifles that launch explosive rounds, rocket launchers that set everything on fire in a 10 foot radius, scoped shotguns that put 8 slugs in a tight circle on target from 200 meters away, SMGs that fire 4 shots at a time and electrocute the target, and everything in between. A good gun can really make a difference in a fight, such as finding a strong sniper rifle early, or getting a machine gun with a +60 round magazine that lets you walk up to someone and put a hundred rounds into their face. Sometimes enemies drop guns, but more often you'll find yourself giddy as you open a sealed gun case and see a gleaming new rifle. Really addictive.
2) Weapon proficiencies. Instead of putting points into weapons skills as you level up, you gain bonuses for them the more you use them. This is more realistic and a lot more fun, since it gives you a way to reinforce your powers through use. Clearing a few caves with a shotgun will quickly net you added weapon bonuses that really pay off. Separately you can also (starting at level 5) put points into your skill trees, so you can get some nice cumulative overlap.
3) Second wind. One great concept is the "second wind" -- when you get KO'd, you get a few seconds to kill one of your foes and get a "second wind" that pops you back onto your feet. This is particularly nice when you get overrun by mutant dogs or flying enemies without warning. In a number of fights, I killed nearly every foe only to have the last couple take me down at close range. With a second wind, you manage to survive -- and it is extremely gratifying.
4) Level resets. One nice feature is that quitting and reloading the game resets all the areas, so you can farm areas repeatedly for EXP, different guns, and the like. This lets you level up if you are having problems, make money, and get those weapons you wanted.
Now, a few negatives:
1) No armor. Your character gets a wide array of selections between guns, grenades, and shield generators. However, there is no other armor in the game. This seems like a big oversight and cuts into the customizability of your character a fair amount. It would be nice to have slightly less guns in exchange for boots, gloves, helmets etc.
2) Level-specific foes. Enemies have a high respawn rate and scale somewhat based on your level. This usually works fine, although you will face a lot of dangerous and accurate enemies. One problem however is that you seem to do less damage against higher level enemies. In particular, if a foe is much stronger than you, your weapons do almost no damage, while theirs will destroy you. This might be an effort to keep your level growth under control, but you usually find out through trial and error. I had a character go up a level in the middle of a fight, and he went from doing 20 damage to some high level raiders to doing over 100. It was pretty strange.
3) The graphics in the game are interesting and appealing, but there are some unfortunate bug effects. Despite a nice strong jumping ability, it is possible to get stuck in the terrain, which can also effect enemies. NPC scripts can sometimes get broken if interrupted by fights or the like, usually requiring a reload (for example, I've had that little droid from the start of the game freeze up a few times, meaning you can't get through a gate and the game is basically broken).
4) There are vehicles you can drive around in, which is another nice feature. They also make good heavy weapons platforms if you are having problems with higher level monsters. However they are also a little buggy -- I had one simply blow up for no reason (and with full shields), and another get stuck on some rocks that wouldn't let me exit and required a reload. And reloads take forever.
All in all, Borderlands is a fun game with much to recommend itself. Popping people with a sniper rifle and then cleaning up with a shotgun or assault rifle is a simple joy that is endlessly repeated. If you're looking for a FPS with character development, or an RPG with lots of fighting, this is your game.