Historians do tend to be lefties
That's because reality has a left-wing bias.
Seriously though, it depends on how you judge Bush. Did he succeed in what he set out to do? Let's take this one by one.
That massive tax cut he basically ran on in 2000? Achieved, but not without difficult consequences for the economy, the results of which we are starting to see now. The historical comparisons to Harding's- and Coolidge's lassez-faire approach and how it lead to a build-up to the economic crisis of the late 1920s is almost too eerie to bear (though I don't necessarily want to jump into the boat a lot of journalists have gotten into and melodramatically scream "Another Great Depression", because that really remains to be seen, and I remain skeptical as to whether it'll be AS severe as a lot of people say). It does bear mentioning that Harding and Coolidge themselves aren't exactly viewed as top-raking Presidents either.
Iraq? You need to wear blinkers the size of Siberia not to view it as a complete and utter disaster.
Privatising social security? Never achieved, while Karl Rove's dream of a Republican hegemony that will last for decades looks to be in tatters.
The world is arguably less safe now than it was immediately after 9/11. Iran is now the biggest regional power in the Middle East, partly because of Iraq.
Bin Laden has not been captured and is likely to be still at large.
Politically, America is probably more divided along partisan lines than since the days of the Lincoln Presidency.
I could go on, but the signs aren't looking good for Bubba. All that said, it'd be too hasty to judge right now how Bush's long-term standing will be. I
personally suspect that it won't be pretty for him and that he will join the likes of Harding and Buchanan as the all-time worst Presidents, but that's me and I have an admittedly biased view as things stand.