Summary: Fun, very casual puzzles that are nice to wind down or relax with. But if you're looking for something very intensely engaging or a massive timesink, this is probably not going to be it.
Very nice to look at whilst still being quite minimal in style.
Unique puzzles that make you think a little differently - to be precise, think very much in 2D.
Relaxing - I like to play Kami when I want to zone out of work or at the end of the day. It's challenging enough to be engaging but not so addicting (for me, anyway) that you want to play it until 2am. At that late/level of tiredness you probably couldn't get anywhere with the harder puzzles anyway.
The music. I just switched it off, but then, it's just not my type of background music. It might be yours.
This is a VERY casual puzzle game with no extras (other achievements besides 'Oh hey, you completed the level, here's the next one', or a story). It's part of its appeal but may also turn people off who are looking for something a little more exciting.