I just got Lucky # Slevin, and I liked it. Witty and funny. Not quite as witty and funny as Kiss Kisss Bang Bang, but still good. Josh Hartnett was better than I thought he'd be. Everything else I've seen him in he's got the dynamic of a plank of wood.
I did get the impression that it could have either been funnier, or more compelling (but not both). To be more compelling it would have needed to be more serious and less slapsticky. To be funnier, the opposite. As it is, it's kinda inbetween, which is probably where the "average" feeling comes in. Performances carried the movie a lot where the story might have languished (yes, even Josh Hartnett).