I've tried the brompton and I bought the dahon curve d3 instead, both fold to similar size, with the brompton being more compact, but not by much. I liked the folding style on the dahon much more than the brompton, I'm not down with twirling those two screws on the brompton monotube and headtube whereas dahon just has a clamp on the monotube and headtube. And dahon was half the price of the brompton. If I had a 20" dahon I would consider the brompton, but I have something so similar I wouldn't bother.
As for the poster suggesting a bike friday tikit, as much as I like bike friday's line up and the speed of tikit's fold, it folds bigger than brompton and 16" dahons. The purpose of a compact folding bike to me is the size, and the tikit is bigger than I like it to be.