Essentially the title. I've made a Chocolate Cherry Stout recipe and I'm trying to weigh the pros and cons of extract vs real fruit. I've never used fruit in a beer so I'm basically in the dark.
I've never had good results with cherry extract -- the stuff tastes like cough syrup. Canned cherries (the kind explicitly intended for brewing, processed to reduce pectin haze) will produce a much better taste. You'll want to use a lot of them in an assertive style like a chocolate stout.
Oh, and remember -- the fruit will provide tartness, not sweetness. In a dark beer, you run the risk of that flavor coming off as astringent. Consider sweetening with lactose for a more familiar cherry-ness.