I have the misfortune of being a PS3 person playing Skyrim, but I am still holding out hope that Bethesda will deign to allow us to play their DLCs for this game. I have read a great deal about Dawnguard, however, and several coworkers have had very good things to say about it, on XBox360. So, when you find Serana, how much of Dawnguard do you have to play before you can take her to other parts of the world, and do other stuff with her (Companions Quests, College of Winterhold stuff, other missions that don't require a specific follower), or dismiss her, and then just go find her again, when you want her, instead of Breylina, or someone? Do you have to do all of it in one rush, or will she follow you around, and let you do Dawnguard stuff as you wish?
She'll start following you after about an hour, but you won't be able to do the stuff you would expect from a follower. After about another hour she will join you as a proper follower and will do everything one would expect, except taking direct commands with "I want you to do something", the option never appears.