Just not that funny
I'm a huge Will Ferrell fan. The scene in Old School where he hits himself in the neck with a tranq dart makes me cry laughing.
I'm not a huge fan of Dubya. Very much the opposite. That said this is really not very funny. I mean Ferrell does a good W but 90 minutes? After 25 minutes I was checking the clock. It essentially recounts his life and high (low) points of his 8 years in office which were horrific for our country and downright embarrassing, and so the comedic material is limited. Making fun of all his missteps, scandals and boneheaded mannerisms winds up seeming vindictive. Unless you are a hardcore liberal wanting to see Bush ridiculed to no end I'm not sure what the point is here other than Ferrell getting the last bit of mileage out of an excellent impersonation.
I had high hopes but really there's just not much funny about Bush's presidency beyond the moron himself, and after about 5 minutes you get it, yeah, the guy's an idiot, and was surrounded by frightening ideologues who took our country over a cliff.