I hate you all for hating on Sera 
(fake hate. Sorta)
I, personally, am not hating on Sera at all, lol. I recruit her and befriend her every time. But, the fact that we can tell her to leave at any time pretty much tells us that Bioware was aware that she, of all the companions, would for the most part be a "love or hate" kind of companion.
Her manner of speech and complete and utter lack of manners, civility, and maturity can make her difficult to like for many and impossible for some. Plus, I'm not fond of how anti-magic she is.
Took her a while to grow on me (same with Vivienne, naturally), and I still only sort of like her. Still, she can be a lot of fun! But, I almost never take her with me. Same with Bull and Vivienne. It's nothing personal to any of them; they just never did anything for me in any way. :V I will still befriend everyone and do their quests, of course! (My "dream team" is Dorian, Cole, and Varric, but I also enjoy Cassandra, and Blackwall's alright, too.)