There are several topics on this issue, so for the newer Dawnguard players we suggest you keep Serana with you, or you'll be re-loading save files.

Here are some known bugs:

• Serana's dialogue changes if you dismiss her, and she will no longer follow you.
  
• If you ask her to wait, same can happen and the only response she will give you is "yes"
  
• When dismissed, she does not show up in your fort (depending on what side you chose)
  
• Responses from her can clue you in to her bugs - such as "I'll wait for you at (Daggerfall Cave)" or any other place than your fort means she has bugged.

If you want to enjoy having Serana as a follower, keep her close. If you want to get rid of her, let her bug out.

Hope this helps some of the lost Serana questions:)

I hope this gets fixed soon, she's one of the best followers: Essential, powerful necromancer, has Expert level Destruction spells, great Sneak, and has the profound effect of making you regret dismissing her. All on top of her unique personality in Skyrim and general behavior. I'll be honest, if 1.7 gets delayed just fix her, then by all means Bethesda go right ahead.

I just dismissed her actually. Now I'm kinda nervous... she said she was going to meet me at the Glade (to get the moth priest's knife). But I haven't met back up with her yet.
Though I did notice afterwards that my game will no longer save automatically (and yes, all my auto saves are turned on). I just started a new game (my old game actually corrupted) so I hadn't yet done the Thieve's Guild questline yet, but I messed up on the goldenglow (I used the fire breath shout) and burnt 4 hives, not 3, and when I went to reload from my previous game, the last save was at the Soul Cairn, and not where i had just fast traveled from (Ft Dawnguard)

I do hope this is fixed soon. I love Serana to death, but a man needs a little space.