It can be a fun movie if you turn your brain off when you watch it.
And yeah Robin seemed way to old to be "adopted" by Bruce Wayne. I don't care if the circus was halfway to Metropolis, there was no reason he couldn't have just hopped on a bus and rejoined it.
Someone above said that Jim Carrey was just doing Jack Nicholson's Joker. Personally I think that's what Tommy Lee Jones was doing. TwoFace just didn't seem to have a clear motivation.
And Jim Carrey was just doing Ace Ventura: Riddler. He can be a very good actor when he wants to be. I would have like to see him do a more serious, darker exploration of that character's psyche.
The love interest was pointless and forgettable. And I agree with the poster who mentioned the complete lack of any self awareness.
Alfred was good though.