One thing that the cartoon effect in Borderlands (both) does, for me at least, is make the suspension of disbelief much easier.
Sure, it takes you maybe a few minutes in the beginning to get there, but once you're there, you're not yanked out of it, because something sticks out like a sore thumb.
Take Diablo III as an example. Blizzard went out of their way to make some amazing looking cinematics for their cut scenes. But that rips you out of your story and then pushes you back into the usual graphics again afterwards.
Compare that to Borderlands, where the cut scenes are, at most, rendered at a better quality than your regular settings.
Also, there's a lot to be said about interacting with the world's deadliest 13-year-old, when the character manages to be both adorable, funny and really scary at the same time.