In the beginning, Morrison's story line was being promoted as a return to Superman's roots, telling tales of a Kal-El far less powerful than he would eventually become acting as a defender of the common man. The first issue started out that way, but that direction didn't last long. The seeds of what was to come were planted in the very first panel with the appearance of "the little man," who would eventually be revealed as Vyndktvx. Then things got cosmic pretty quickly as Brainiac showed up at the end of the second issue. Following the defeat of Brainiac and Superman's discovery of his Kryptonian heritage, things start to get truly weird and Morrisonian, as the story of Lord Vyndktvx's revenge moved to the fore. At first, the villain only made cameo appearances in what appeared at first to be a series of stand alone one or two issue stories. With #14, though, his plot against the Last Son of Krypton swung into full force.
This is definitely a story that reads better when taken in all at once rather than in monthly installments. Reading the last few months worth of issues as they came out, I really couldn't make heads or tails of them. However, when read straight through, the pieces of the puzzle fit together neatly to form a surprisingly straightforward tale of vengeance, albeit one played out across all of time and space and through various alternate dimensions and planes of existence. Despite taking an extra issue to wrap things up, the ending still in spots comes off a little rushed and disjointed, but that's actually pretty typical of Morrison's epic super-hero sagas. Despite its flaws, the final chapter was a fittingly grand conclusion to Grant's run.
It does seem to a certain extant that Grant is having his cake and eating it too with this story. While it reintroduces a new Superman for the New 52 continuity, it simultaneously draws on the past history of the character. Certainly a working knowledge of Superman's past encounters with Mr. Mxyzptlk helps in order to get the full impact of the tale.
On the whole, Grant Morrison has crafted in these nineteen issues a Superman story in the classic mode, pitting the Man of Steel against a foe truly worthy of his might and once again reinforcing his stature as the world's greatest hero. If the run failed to live up to some people's expectations, I would say that is likely due more to the fact that, spurred on by Morrison's earlier All-Star Superman, those expectations were almost impossibly high, rather than any deficiencies of Morrison's story itself.