Tap 6 and tap Highlight, and you can see all the places a 6 could go. Handy.
For mere grasshoppers in the ancient ways of sudoku (also known as "newbies"), Satori Sudoku is helpful and forgiving. If you know what you're doing already, you can just tap an empty box, then tap a number in the keypad: once to make it a notation, twice to make it your real guess.(We'd prefer it be one tap to guess, two taps to notate, but that's a small quibble.)
But if you need some help, you can tap an empty box and then tap the Auto button, and the game will highlight the possible numbers for that box in the keypad. Or, tap a keypad number first and then the Highlight button, and the game highlights all the empty boxes that number could possibly fit in.
As for difficulty levels, you only get Easy, Medium, and Difficult. There’s no timer, and you can’t enter puzzles you see in the newspaper. But the interface is uncluttered, the help is helpful, and there's no music so you can listen to your favorite iPod tunes instead.