I've always suspected that he was significantly effected by substance abuse, but that he probably was not naively stupid.  Un-interested perhaps.  He was in the 25th percentile in terms of aptitude when Papa Bush got in into the Texas air national guard to keep him out of combat yet he graduated around the middle and was briefly able to operate a military jet aircraft (unless that was stagecraft which I would not rule out.)  That says something to me about his capabilities.

As prez, GW had a 6 year string of getting everything he wanted as soon as he wanted out of the Dems so it's funny to hear them call him dumb.  OTOH, he was an installed puppet who probably did little or no strategy or planning on his own.  Most presidents of modern times have been this way, but GW was among the most obvious in this regard.

An entertaining hypothesis I ran across recently is that GW's grandfather through his mother was Aleister Crowley.  That would be hilarious.