I liked him in Beetlejuice, but I'm pretty sure his reputation as a comic actor lead people to shun the film.
As for Keaton actually AS Batman... He was okay. He had an awesome Batman voice, but other than that, he was VERY bland. I'm also not a fan of Burton's Batman, especially Batman Returns (quite frankly, a terrible film altogether).
For me, the best Batman so far was Christian Bale. He had the right appearance and physical ability, and played the detached Bruce Wayne flawlessly.