Whatever you do, don't buy this expecting Fallout 3 with better shooting. Its a shame to say that Borderlands is simply far too basic in both RPG terms and Shooter terms to satisfy either craving particularly well and thus the whole games never quite feels comprehensive.
If you're approaching this expecting a decent RPG then prepare to be disappointed. Choosing from a base of only 4 characters with almost zero options to customise (although you can change the colour of you're uniform - essentially pointless in a FPS where you rarely see your character) you're plonked into a world thats entirely static and unpopulated. You can't enter buildings, you can't search random crates or storage points (everything to be found is either dropped and highlighted or in highlighted special boxes), you can't search the bodies of bandits or creatures, you can't combine items or make items, you can't find interesting clothing, food, random things - pretty much everything that you might expect to find in an RPG is absent. In fact you won't be able to find anything that isn't a gun, ammunition, weaponry modifiers or money and having played for around 8 hours now I've only interacted with a meagre 3 people. The towns are empty, the shop is empty, all buying and selling is done via machines dotted about and the result is a world that feels oddly empty and devoid of life. The few NPC's you do encounter are sat immobile in one location, never moving, purely there to give you the quests. And sadly pretty much every single quest is a 'kill this fetch that' affair with little or no imagination. In fact the bulk of the early hours involve traipsing back and forth through the same locations killing the same enemies - the ubiquitious Skags that infest every single location and come in every flavour from easy to kill to stupidly hard to kill.
It can't really be understated how empty and dull the world is. You have a goal to reach and thats it, theres no side tracking, no exciting discoveries, no collecting anything thats not directly related to your quest. The landscape is devoid of places of interest, consisting mostly of rusty outposts full of bandits and skag holes in every cliff face. The land is also not continuous, being divided up into areas with a loading screen inbetween. Each area is fairly small, meaning most quests can be reached within a minutes walk from the empty local town. Theres simply no sense of scale or grandeur, no sense of setting out on a perilous mission.
So really the only possible link to an RPG is that you level up as you go, which allows the spending of a skill point to be assigned to a slot of your choice - upgrade shield, improve melee, etc etc. Hardly comprehensive for an RPG.
So. Lets discount the RPG aspect and look at Borderlands as a shooter. And whilst it does have the capacity to be entertaining, generally its far too basic to succeed purely on those terms either. Combat is utterly simplistic - enemy AI is non existent, so most foes run straight at you every time. You'l spend a lot of time running backwards shooting or circle strafing, banging away until the enemy, usually some manner of Skag, falls over. Its terribly unsubtle, and frankly repetitive. For a shooter, as before, the world is far too static. Cover is limited and non dynamic, theres no physics objects to chuck about - every fight requires pretty much zero consideration to how you interact with the environment. In some cases its easier to stand there, take the hits and simply pump shotgun rounds into the bad guys.
Its all completely lifeless stuff for an RPG and cripplingly repetitive, lacking character and intrigue. Collecting loot is fun, but not really enough.