Personal opinion here, Borderlands games have a solid story line, though the drama and plot twists don't pick up until the second and third games. All the games are riddled with edgy humor which gets a little repetitive at points, but is generally very amusing.
Graphics were very original when the first game was released, and they're stylish and fun.
Lots of customization with four base classes in each game and several more added as DLC, each class having multiple skill trees which can greatly alter your gameplay experience, which is a plus for replay value. A strong detractor from the replay value however is that you cannot skip lengthy cinematics, or bypass tedious non-combative storyline quests; these issues are exacerbated when you start tackling the harder difficulty playthroughs. If you make it to the late game, you'll find that most of your skill trees have become impractical to use, forcing you to re-spec into a cookie-cutter build and farm specific legendary weapons so you can abuse their unique properties to stay competitive, and even then you'll spend every minute praying that an enemy doesn't decide to spawn right under your ass with a shotgun, or get a lucky sniper shot from across the map killing you in one hit with no enemies around to kill so you can recover. If you find the gumption to keep going despite all that, you'll find that the level grind is extremely slow and absolutely required to progress.
Multiplayer, the server list doesn't have enough information so you'll spend half your time joining games with people on different quests than you, and the other half joining games with hackers who run around 1-shotting every boss totally ruining the experience for you. Of course these issues can be ignored if you play exclusively with friends.
I love the Borderlands series and will definitely get the next game when it comes out, but these issues definitely knock a couple stars off the series' score for me and I hold no hope of them changing any of them going forward.