The President has one thing in common with his predecessors: He claims to not care about his legacy. Most seem to say that at one point or another; in this case "[w]e are still arguing about the record of the first president...I'm sure they will take their time when it comes to judging my record."
It is one of the more benign lies he has told, maybe because it only reveals his comprehensive inability to understand history. There is no harm in that kind of ignorance, though it has grave implications when it comes from your leader.
Of course, I would love to know what exactly he thinks we are still arguing about with Washington. Does anyone think he was a terrible President? His farewell address is universally regarded as a gift to the country, especially its call to avoid foreign entanglements. Leaving office after two terms and helping an orderly transfer of power to a successor was a move of great courage considering the former general could have strangled the republic in its crib if he had wanted. What are we arguing about again? And of course even if it were true he neglects to consider more recent Presidents like Harding and Hoover whom everyone pretty well agrees were terrible. Not too many arguments there. Contemporaneous opinion about Presidents tends to hold up, which makes someone like Truman a very rare exception. In other words, don't expect your stock to change too much.
Of course, he clearly does care about his legacy. I'm no fan of his so maybe the following is unfair but here's a quick review: No Child Left Behind (mixed results, likely to lapse); tax cuts (likely to lapse); surpluses into deficits; Hurricane Katrina (botched response, no follow through); 9/11 (failed to unite nation with sense of shared purpose and sacrifice; told us instead to keep shopping); the economy (bad and getting worse). The only potentially bright spot is Medicare Part D. In foreign relations he started the Afghanistan war well but has handled it poorly since; alienated allies; was a credulous boob with strongmen (Putin, Musharraf); aggrandized enemies (Iran and North Korea, though the latter has greatly improved); and astounded the world by actually making the Israeli-Palestinian conflict worse. And of course Iraq.
All the problems above are stubborn in some way. In the time left there simply isn't a lot of room to change them fundamentally. He can nibble at them now and maybe change course a bit but by and large he won't be able to turn those battleships around. Executive power is another matter. He has made extraordinary claims and the question now is, what will future Presidents do about it? He may fairly echo Ben Franklin when the next one asks him "what have you left me?": "A unitary executive, if you can keep it." Will the next one claim executive privilege to cover up any politically uncomfortable truth, or make extensive use of signing statements to justify not following the law, or inflate relatively modest tools like Status of Forces Agreements into robust ones like treaties? The candidates aren't saying at the moment, but unless they explicitly disavow them (as John McCain has on signing statements) we should expect them to happily do so.
Much more interesting is the gray area between executive power and lawbreaking. This year the President's top legislative priority started out as getting the Protect America Act made permanent. That seems to be off the table, so he scaled it back to just telecomm immunity. As others have noted the priority now is clearly to not let what he has been doing ever see the light of day, but even his allies acknowledge "his political capital is waning". The lawsuits against the telecom companies will likely have explosive revelations about what the President ordered them to do. The great problem for him is that his activities are not the subject of the suits, but potential evidence against others. Once discovery begins it all comes out. At that point the damage is done - the political backlash will likely be immense and immediate. The verdict doesn't matter, only the airing of secrets. The same is true for additional details on our torture regime, the retrial of Joseph Nacchio and the activities of the CIA that seem to require wider and wider insurance coverage. Revelations in these and other areas promise to continue to emerge long after he leaves office and he surely considers that a mortal threat to his legacy. He wants to put them beyond the reach of inquiry before he leaves, and that may consume more of his energy than anything else. If he could take every shovel full dug out for his library and dump it on these inconvenient secrets he surely would.