Grant's a songwriter has-been from 80's band Pop (read: Wham) who hasn't written for a decade. He gets the gig to write a new song for Haley Bennett (read: Shakira) in, oh, six days. Problem is that he can only write music, and needs to work with a lyricist. Into his life walks new plant waterer (really) and stream-of-consciousness motormouth Drew Barrymore, who randomly rhymes some words in his apartment, and gets taken on. Shockingly, Chemistry ensues.
In terms of the story, this is nice enough. Grant does his usual thing effortlessly. I don't think he actually says "gosh," but it's a close-run thing. Barrymore's engaging enough. Neither has lines that have the wit and sparkle of Richard Curtis, but it's not bad. And the 80's send-up bits are very good, as is the D-list celebrity life (c.f. the start of Galaxy Quest).
Where this falls down is, ironically, the music and lyrics. Songs are actually performed by Grant, Barrymore and Bennett, and none of them have voices that are strong enough to carry a song. This isn't a problem for Barrymore, who isn't playing a singer, or for so much for Grant who technically is supposed to be a writer rather than a performer (though he's making his living via nostalgia shows), but is a big problem for Bennett's character, who's supposed to be someone to rival Britney, Aguileria, etc. Those women have amazing voices, and Bennett has a really weedy one in comparison.
Then there's Barrymore's lyrics, which are better than, say, Fezzik in The Princess Bride and Pam Ayres, but not by much. They rhyme, and that's all that you can say about them.
And finally there's the music, which is sub-Disney-knock-off. No, wait - they're about the level of X Factor winner singles. They're that bad.
Which is a shame, because they distract from the rest of the movie. A good song can lift a movie above the average script, but a bad song drags it down, and especially when the song's supposed to be a pinnacle. Think of Bill and Ted producing the greatest song ever, the one that's supposed to lift mankind to a higher level, and it turns outs to be God Gave Rock and Roll To Us by Kiss.
So, entertaining, with gritted-teeth bits. Bring back Richard Curtis.