This is something I wish more companies would do with their free products.
Is it just removing ads? Or is it a premium product?
e.g. Gmail has no ads when you buy a Google apps account, but you pay $50 for it because they give you a load of other features as well (Which you may or may not need).
I still think it's good to give the option, but your selling points could get diluted pretty quickly once you start adding more little things to justify your paid memberships.
Of course that is likely an issue with the advertisers that choose wrong keywords that their ads show up for.