Its not that good. Its a bit overly long. Pretty entertaining up until he meets BB, then its a bit of a slog. Not even that interesting from a biographical point of view as it focuses mostly on him being a bit of an arsehole. You see him in the recording studio only once so its pretty unsatisfying from a nerdy music point of view and weirdly it doesn't show any of his kids being born, as if we're not really meant to care. Spends about 10 minutes at the start on him as a child living under occupation rule, which potentially could'ave been interesting but wasn't really.
There's a quotation at the end from the director saying something like "i was never interested in the real gainsbourg, the myth was always better than reality and thats what i tried to show" which works a bit in the context of the film but seems like a bit of a cop-out.
Still, regardless of all that visually its really cool. There are 2 big puppets which follow him around and they're really well done (although the main one is basically his devilish side which tells him to cheat on his wife and be a dick the whole time, which is a little patronising). The acting's good, too, and all the the chicks are attractive if you like that sort of thing.
Anyway, its worth your attention if you like Gainsbourg but try to travel into the past, get the guardian from a few days ago then get free tickets for an advance screening like I did. Or sneak in. Or get a 2 for 1 deal or watch it on the internet because its probably not worth £7 or whatever you pay for the cinema these days.