I love Sera.
She's irreverent, she's a *****, she's rude but she's freaking funny. I also love how she slowly opens up over the length of the game. Her voice also reminds me a looooot of Maise Williams (actually looked it up to see if it was her) so that's another plus.
I only just got the Cookie scene and it was in my second playthrough. Strangely enough, she seemed to get along a lot more with my Human Noble than Mage Qunari . Well, it's a different relationship I guess. With my Qunari, she was really like a little sister to him. I had good enough approval with her but I was almost always chastising. There are shades of that with Human Noble but he's a lot less judgmental. They're more akin to buddies, I guess. It helped that her most contentious scene (the one were she ordinarily kills the noble) was able to be resolved relatively peacefully because my HN had the Nobility Knowledge perk. There were less rough spots as a result.
Her banter with Dorian, Cassandra, Bull and Blackwall did a lot to endear me to her too.
Granted, her attitude towards other elves can grate quite a bit. I found a couple of her exchanges with Solas funny but she can a bit too much in that regard.