I have had both wonderful and terrible experiences with stouts fermented with different yeasts.
Dupont Monk's Stout is a favorite of mine (I almost prefer it to their Saison). Stone's Belgo-Anise IRS, however, I was not pleased with.
I'm thinking about WLP670, witbier yeast, Roselare, and possibly trappist high-gravity as an experiment.
Has anyone else played with funky yeasts in dark, roasty beers?