I hate sudoku. That being said, this seems to be a nearly-perfect presentation of it, and sudoku seems to be really, really popular with a lot of people, so I don't see how one could reasonably not give this a high score.
Generating puzzles that actually meaningfully vary in difficulty between levels as well as implementing a convenient and functional note-taking system makes this have just about everything a sudoku lover could want (I imagine). The only two complaints that I can come up with are the fact that the note section limits the player to seven characters per square and that shift moves the cursor out of the note section, making capital letters and some punctuation unusable, but those are barely worth mentioning.
The only thing I could even imagine adding is a "solver" tool which not only tells you the next easiest unsolved square to figure out in the puzzle, but also how to go about figuring it out. Before writing this review I tried to complete a puzzle of each difficulty level so I could be reviewing the "whole" game, but after spending like two hours staring at a puzzle of the highest difficulty and only getting two squares solved for sure, I had to give up. Part of the reason I hate sudoku is clearly because I'm terrible at it, but I am willing to learn, and after taking all the notes I could on the puzzle and then guessing at many individual square results to see how they panned out and then even a few guesses at pairs, I basically had no idea what a next step would be. So, yeah, really all that I can think of that would be worth adding is something that basically teaches the player the proper technique to solve a given sudoku puzzle when they are stuck, and clearly that's beyond the scope of what this project is, so I can't help but give it a perfect score.