The Brompton is the unofficial bike of the FNRttC, and there are usually a fold of hardy Bromptoneers on the rides. I keep saying that one day I'll do the Brighton ride on mine (on account of it being relatively short and hilly, which would seem to be the least worst conditions for a Brommie), but I need to be fit enough that I won't spend the whole ride swearing about it being a stupid idea and that I should have brought a proper bike.
I'm firmly of the opinion that Bromptons are a brilliant solution to things like avoiding getting your bike nicked and BloodyTrains, but far from ideal for rides of more than an hour or two.