I went from a school in special measures to an all boys grammar school from the transition to GCSE->A-Level.
I can quite honestly say I've enjoyed it. There are grades I never thought I would be able to achieve. The school I go to embraces external students and doesn't discriminate against them. Whilst they like you to read ahead and be able to achieve full marks in every assessment, the staff actually care and will spend their time to help you move from that awful E to that amazing A.
The people are somewhat stuck up but most of them are really polite and friendly when you get to know them. Some people think that they're "smart" just because they were in a grammar school in Y7-11 but you need to remember that you had to work harder to get in there!
I think you should give it a go, it really brings out your true colours and you can see yourself in many different ways. I do not regret going to a grammar school.