Papa Haydn serves excellent food and dessert, and the environment of the restaurant is fun and sophisticated. The neighborhood (NW 23rd Ave) is fun, and I think adds to the appeal of the restaurant.
The food is delicious and creative, and you have plenty of options. The desserts, though, are a major draw. The cakes, the panna cotta, anything with chocolate -- Papa Haydn knows their desserts and offers a wide selection.
Expect a bit of a wait on weekends, but the wait is worth it (and save room for dessert!).