It's a really nice add on. I'm in love with Serana, but you can't marry her D:
She's a great companion to have though. Oh and there's more playtime to get from each story since there's also side objectives. Mostly just "hunt down, kill this person" kinda stuff but if you take Serana along it feels much better. Plus with the new weapon additions it adds much more fun. And I mean TONS of fun (blacking out the sun is pretty fun, I don't want to say much more other than that though unless you ask since it's minor spoilers).