I love Val Kilmer. Seriously he's been great in so many things, and I have a soft spot for really eccentric types. Doc Holliday, Jim Morrison, his work in 'Felon', 'Heat', 'Kiss Kiss Bang Bang', 'Joe the King', etc. Lots of good work.
That said, I don't think he was a great fit as Batman. But who knows, perhaps that was the fault of the absolutely astonishingly bad movie it was. Think about it, Batman as a character is usually irrelevant to the film as a whole. He has so little to do with the movie (stonefaced dude in a mask, come on now), that Batman films are made or broken by the story and the villains and supporting cast.