One guy from the New York Times cited the scene where Sebastian plays a synth solo during his live performance with his new pop-jazz fusion band as an example of what annoys him about this movie:

"The scene that best captures all that irritates me about “La La Land” comes when Mia goes to see Sebastian, who has scored a gig in a pop-jazz band fronted by his old frenemy, played by John Legend. Seb plays a blistering synthesizer riff, and Mia recoils and flees in moral horror at his selling-out, as if he’d just carved up a baby onstage. If playing an awesome synth solo is wrong, “La La Land,” I don’t want to be right."

This is yet another example of the wrongheaded approach people are taking to this film. LA LA LAND's substance has nothing to do with the music itself, just as that scene has nothing to do with Sebastian's synth solo. Damien Chazelle is *not* telling us what to like. He has nothing against Sebastian's solo. Mia does not "recoil and flee in moral horror", she gets shoved and left behind in the enormous crowd of people who are connecting with the music, who are there for the music, while she's merely there for him and can see that what he's playing isn't something that he's connecting with. She is literally and metaphorically distanced from him. I'll say it once more: Damien Chazelle isn't knocking Sebastian's solo. His movie has nothing to do with the supposed quality of the Jazz Sebastian loves, or the synth solo that he plays. And the film doesn't even portray him as a sell-out! The only reason Sebastian winds up with the Jazz Club of his dreams is because he swallows his pride and joins Keith's (John Legend) band, despite his PERSONAL distaste for the music. Mia actually later confesses to Sebastian, truthfully, that she does like the music he's playing; she's just concerned that he doesn't. And she is right. Not because what Sebastian likes is better, but because it's what inspires him, and is part of what inspired their love for one another. It's Sebastian's passion for and intimacy with both his music and Mia that concerns her, not the synth solo. It's what the synth solo implies about Sebastian, not about Jazz. Chazelle is a humanist, people. He's not the finger-wagging, naive Jazz obsessive that he initially portrays Sebastian as. This scene expresses a mature yet problematic development in Sebastian's character, and more importantly, in his relationship with Mia: he has swallowed his pride and finally found a successful gig for himself, but at the price of what drew him and Mia together in the first place: their mutual passion and love for their dreams and aspirations that eventually manifested itself in their love for each other.