It's a nice concept, though hardly an original one: fantasy musical numbers are intercut with "real life" in order to show the inner workings of the characters. It's also a Hollywood Golden Age tribute movie. It's also a classic follow-your-dream tale of an aspiring actress and musician in Los Angeles (LA-La, see?). The problem is, it doesn't do any of these things with enough skill or conviction.
The opening sequence, a dance in a traffic jam, is very pretty and inventive but pretty much irrelevant to everything that follows (a shame, because the singer introducing it is the most musically pleasing thing in the film and you never see her again).
It's very hard to care about Emma Stone as the heroine because you don't really spend time getting to know her before you're supposed to be rooting for her. She worries that she may be just another wannabe and that she doesn't deserve to be a star, and you kind of feel she's right. Which would be fine in the context of another film, about the gap between hopes and reality. But it's very obvious that that's not on the cards for this plot.
It's a shame that so much money and time went into this and so many tricks were missed. The choreography is OK, not as good as it could have been. The singing is OK, except in the case of Stone (good dancer, big eyes, amazing balance on high heels, really not a singer.). The dialogue is OK, sometimes very good, rarely bad.
Lots of avenues are half-heartedly explored in a way that is very frustrating because you see what potential the film had. Hollywood writers are briefly satirised by a talky party guest, the heroine's housemates form a sparkly quartet for an early number, visual nods are made to Funny Face, A Star Is Born, Le Ballon Rouge, the Bandwagon, An American In Paris etc - but none of this is sufficiently dedicated or consistent to hold the film together. It's a series of non-sequiturs.
There are also some bafflingly obvious continuity errors: Stone walks to work in one outfit and leaves in another, for example. And how do the pair get private access to an observatory after hours? And you're supposed to feel sorry for Gosling's jazz musican because "jazz is dying" and no one wants to hear him. I'm sorry? Tell LA that jazz is alive and well in the rest of the world.
The acting is the best thing about it - the rows and the banter and the final climax sequence are brought alive with professional skill by Stone and Gosling. It's the kind of time-filler one might watch on a plane, but I wouldn't bother to watch it twice.