As a PS3 user I have yet to meet Serana, but after hearing so much about her I'm going to be a tad disappointed if she isn't the greatest NPC ever.
She's alright. She's no replacement for Derkeethus, my lowly Argonian squire and loyal sidekick. Though she's less inane and chatty than Mjol The Lioness. What's that you say, Mjol? You miss your father? He wanted you to be a hunter? Thanks for the update, be sure to tell me again in five minutes.
She has a few cute/funny lines, but I hate her zombie pets. Especially when she reanimates a noisy one, like a spider.
*SKITTERSCAMPERSCAMPERSCAMPERGLORPGROSSSOUNDSASPIDERISFOLLOWINGUS*
No...I won't let you keep it, Serana. No, I don't care that if followed you home. Go find a nice quite bandit to reanimate like a normal person, okay?
She will interact with things like chairs, alchemy tables and the like. This is also fun, at first. Until I need her for something and she's in the other room eating bread with Gunmar. A good lackey must come to heel the instant I snap my fingers.
I'm sure Derkeethus enjoyed his vacation while I explored the Dawnguard story line with Serana. Heavens know I didn't let him return home to his family after I rescued him from that falmer cave, but it's over now! Back to work! To Oblivion and back, as they say.. And carry these dragon bones for me, for goodness sake. Good for nothing argonian.