For a game that attempts to be a standalone RPG/FPS experience, it's pretty bad. The world is barren, you always follow the same linear progression despite what class you pick, every NPC is a quest dispenser with no personality, and any personality the game has is forced meme-fuel that stinks of bad writing.
If you like MMO quest systems, walking around collecting $2 off the ground, a useless loot system, and shitty gunplay... then Borderlands is great for you. If you're above all that shit then just walk away because this series is pure garbage.