I think Clooney could have done a good job had the story not been so campy. I mean, he basically is Bruce Wayne in real life and I think he can pull off serious and brooding when acting as Batman.
I agree, I really think Clooney could've been the definitive, perfect Batman -at the very least the perfect Bruce- if given far better material to work with. When he's spouting off lines like, "Hey Freeze. I'm Batman," it's hard to see it working, but I really think it could've. Him doing Bruce/Bats in the Nolan films (when Clooney was younger) would've been very interesting.