Probably not me. I didn't think his previous show was especially entertaining. I don't watch TV in that time slot much but I usually found myself gravitating to Leno, at least for the monologue and his first bit - with switches back and forth to catch Letterman's Top 10.
I really don't understand NBC's thinking but I'm not a student of late-night TV, so what do I know? Well, other than they just gave Letterman a whole lot of viewers. I doubt that Fallon's going to have much appeal to the people who actually liked Leno.
Don't look at me, I just work here.