I haven't myself, but I've worked at a company that did. Here's what I picked up: BlackBerry users tend to use their devices a lot and are not afraid to download things.
It's not going to be nearly the same market as the iPhone reaches, so iFart and the like would, IMHO, fail miserably on the BlackBerry. I bet there is money to be made in apps targeting certain verticals where BlackBerries are common. Law, real estate, and investing come to mind.