Very straightforward and easy to use. The keyboard shortcuts make sense. The puzzle packs are sold separately but each one is absolutely worth it. ***The puzzles lean towards a more advanced level of difficulty***, so I would absolutely recommend starting with the "Approachable Sudoku" pack. I would NOT recommend this as someone's introduction to sudoku, period. A few minor criticisms:

- Certain custom rule explanations are a bit confusing or poorly worded (there are some r/CrackingtheCryptic threads with clarifications if you've found this comment; search for the exact puzzle name there).

- The color themes don't change the colors of various symbols, which can be confusing. For example, I found that using the only "dark mode" theme made the fortress arrows effectively invisible.

- Color accessibility should be improved. Especially with the lines pack where the colors of the lines are important. I'm not sure if my colorblind friend would be able to play certain puzzles

The free puzzles are good, and the packs you buy are generally good as well.