I can't say I've had that much of a problem with her. But now that I've finished the MQ and I don't think she is marked as issential anymore, I'm going to get Erik back. I only upgraded her weapons, she didn't even wear the armor so I sold it. I liked not haveing to worry that she was going to die in every fight, just take a knee. Plus hearing different comments was nice.
Actually, Serana is the only follower in the game who is permanently essential. Which is the ONLY reason she is still with me, she's killed Vilkas, Lydia, Iona, Vigilance, Shadowmere, Meeko, three trolls, Marcurio and Illith. The DB Initiate and her got into a fight that lasted about twenty minutes until I used a strong enough calm spell on them. I switched my entire build over to an Illusionist in order to deal with her attitude problems.