Mostly very good

On the plus side: the levels of difficulty are great. When you choose one, you know what to expect.
The settings allow you to pick your own style of play, with the level of help you need, from pencil marks to colors and hints that take you to the solution step by step.
You can also choose the way you enter digits, if you want a clock or not; all in all, it is very flexible and easy to use

The pay version is great value for money if you are a sudoku addict.

On the minus side, I found the learn section very weak on the advanced techniques. I had to turn to other sources to understand and learn to use the really complex solving methods.

Lynninaustria123 about :) Sudoku +, v5.2.1