Some leftward Democrats see Obama as betraying the progressive principles they thought he held. Republicans label him as far too liberal then and now. The media like to talk (the all-news channels ad nauseam) about his flip-flopping. Some think that he is pure pragmatist who wants to win, principles be damned.
Here is my tentative hypothesis: Barack Obama is most comfortable toward the left. His core principles are liberal. But he has promised a new politics that rises above crippling partisanship. This involves making compromises, accepting the better when you cannot get the best. He realizes that politics is the art of the possible. He recognizes the futility of being a martyr for lost causes. In all of this there are elements of of pragmatism and realism -- you cannot exercise power for the good unless you achieve it.
Viewed in this way Obama has been consistent in the deepest sense, although his approach involves adjustments in policy over time that give the surface appearance of flip-flopping. This approach puts him in the best light rather than in the worst as his progressive supporters and his avowed opponents are wont to do.
So far he is doing a good job of finding a proper balance between idealism and realism and between principles and pragmatism. Until compelling evidence renders this judgment null and void, I am sticking to it.