"You're a star!" is fairly common in both emails and spoken British English. There are some constraints on its usage, though:
1. It's mostly used by people under 40.
2. It's very seldom used by one male to another male.
3. It's characteristic of office life, rather than home life. (It would sound a little strange at home.)
Possible context: you email some information to a colleague. She emails back: "You are a STAR!" or "You are an absolute star!!!" or even "You are a *!!!!!".
(For some reason, the kind of people who use this phrase are also the kind of people who favour multiple exclamation marks.)
MrP