I love Assassin's Creed. The original is a bit lacklustre at times, especially when juxtaposed with AC2, Brotherhood and the upcoming Revelations, but the setting and gameplay is different enough to most other games out there to make it well worth your while. To me, it was a completely new experience, something that can be quite hard to find in the gaming world.
Basically though, the sequels are better in every single way in terms of gameplay, and the story becomes more and more involved and engaging as you progress. I did prefer the Middle Eastern setting of AC1 to renaissance Italy, but the characters in the sequels (especially Ezio) are much more developed than in AC1 (for example, Altair, who had a U.S. accent for some baffling reason), so it all balances out.
Borderlands on the other hand is a lot of fun when played cooperatively, but it doesn't have much of narrative... it's more a pseudo-RPG quest for kit. If you like that kind of thing, then you'll probably like Borderlands more... but the story in the AC franchise is what sells it for me. They just keep getting better and better.