The plot is excellent, but the characterization suffers.
The characters don't get fleshed out enough, except for maybe Harry, Sirius, Snape, and Draco, later on.
The problem is that everyone seems to fade in and out of focus, with some conversations and character screen-time cut too short, just before we can truly get into them.
Nobody is truly badly written or glaringly out of character, but there is room for improvement in that area.
The author appears to focus more on the plot than on characterization, which results in an excellent plot but also in characters that could be better.
In the author's defense, it is a very ambitious plot.
The humor ranges from tolerable to truly laugh-inducing, but it sometimes keeps the story from being serious when it should be. The writing truly shines when the author restricts the humor and focuses more on serious characterization and pacing.
Also bad is that Harry's mistakes don't cause him the trouble that they should. Somehow, everything keeps turning out all right in the end, when he should have gotten in real trouble many times over by now.
This might still change in the future, though, since what we're seeing is just the setup of the story.
Also of note is that the story needs more proofreading. Some parts are virtually perfect, while others have chopped off sentences, typos, some grammatical and spelling mistakes, and a pacing that is a little off. These appear to be consequences of time constraints on the part of the author rather than an actual lack of skill, but they are keeping the story on the level of Good, when it should be Excellent.
The story as a whole has improved noticeably in the later chapters (it has thirty-six chapters and a prologue, as of this review).
As a final verdict... It requires more patience than it should, because it's a better story than it initially appears. You have to shoulder on through many so-so moments to see the best parts, and it's a little maddening that with more free time (and a good editor), the author could fix it up to be truly incredible, like it deserves to be.
Despite the harsh criticism, I genuinely recommend the story. "Do-overs" are rarely written this well.