I think a lot of this confusion comes from what appears to be a mismatch between how he was when he was up and coming, vs. how he was in his later time as president.

If you watch his older speeches and ad-hoc interactions, he was extremely sharp. And if you watch him later he seemed easily confused and steeped in dogma.

I almost feel like he was poisoned, or beaten down, by the pressure of the presidency and the various narratives from Rove, Cheney, etc.

Either way, there's likely truth to both accounts--that he is both much brighter than people think he is, and that he often acted like a complete idiot.