Love it. Don't think it's posh at all. It would be a serious contender for us if it wasn't for the fact a very good friend of mine just had an Imogen before Christmas :)
She's a prison officer by the way, and her DH is a policeman. Their son is called William. Not posh, not trying to be posh - and the name is pretty high in the popularity stakes (not really really high, but high enough not to be posh or niche).
So anyway, I think it's a gorgeous gorgeous name :)