It's become the Sistine Chapel syndrome: Michelangelo spent a hell of a lot of time on the piece, and it's enourmous- thus, people feel compelled to be awestruck by it, and claim it to be one of his best works. But, in comparison to his sculpture or his smaller works, it's really nothing.
People seem to overlook its compositional or artistic merit, simply giving it credit for how much patience and work was put into it. That's fine and dandy, but I often find myself much more drawn to more pinterly pieces, works that really keep and take advantage of the underlying form. Ah well.