Love it. Our nearly 3-year-old Immy really suits it. Don't think it's posh.
Name books vary, but as far as I can gather Imogen as it is now is taken to mean image but prior to that it was Innogen (Shakespeare is thought to have misspelt it) from the Celtic for maiden.
Her full name is Imogen Sophia, so I really like the fact that it basically means image of wisdom!