I never played Borderlands, but it sounds like I would enjoy it! I did most of the missions available myself on the beta but the final one I had to do with 2 other people as it was very difficult. It was a bit annoying as one guy didn't seem to realise getting blown up by a huge spider tank thing (called Devil Walkers, I think) means instant death, so he didn't bother trying to dodge its attacks and just kept dying... I think the MMO elements are going to be alien to a lot of standard FPS players just expecting to come on and shoot things, so getting any sort of teamwork going might be tough. I'll probably play by myself a lot unless I have a couple of friends online I can party up with.
I tried out a second class today, made myself a Titan and I can now see why I wasn't doing so well in the Crucible (player vs player) as a Warlock! The classes seem balanced but I think I'll need to learn more about fighting as a Warlock if I choose to be that class in the full game as they don't seem to be quite as good in close combat as Titans.