Superb.
I played the first one for a short time and it was a pretty fun shooter/rpg. I stopped playing it because I got side tracked on another game and when I came back to Borderlands I just couldn't get back into it. Not the games fault though.
It would be a bit of a stretch to call Borderlands an RPG *or* open-world. You gain levels and upgrade skills, but they don't necessarily make a huge difference, and it's entirely possible for a character to max out all of their skill trees. It's split into several large areas, kind of like STALKER if you're familiar with it, which you can move around relatively freely once you gain access to move forward, but there's not much reason to return to regions you've finished. Mature themes are there, but they're not excessive, as the game doesn't take itself nearly seriously enough for that. The game entertained me enough to finish it, but I didn't feel motivated for replay; the characters aren't that different and there's no real gameplay variety. The shooting is solid though and if you like the core gameplay enough there's plenty of room for replay value. Haven't played the sequel to compare.