Maaan, now talk about a good puzzle! It really made me think my way through, and it was awesome all over.
This game has pretty nice puzzle mechanics, based on simple arithmetic principles, but making for brilliant challenges. In every level, you have to "link" together numbers on a grid to form the required sums. When you're done, no numbers should remain on the board and, in certain levels, you need to fill in an entire area of cells with your grid movement. While most numbers can only be used once, the game eventually introduce numbers that can be used by a multitude of sequences at once, and that's when things start getting complex and interesting. The game provides us with challenge after challenge, and we occasionally have to crack our skulls a little bit to get some of those working.
Another thing I like is that this game teaches the player about each type of grid tile in a "show, don't tell" manner, meaning that it favors level designs that lead the player to learn about mechanics instead of a bunch of text explaining what each thing does. This is a lot better than reading a bunch of text.
If I were to give a hint to people who are about to play this game, it'd be to keep the following obvious observations in mind: first of all, in levels without reusable numbers, the sum of the remaining number tiles is always equal to the sum of non-cleared "goal numbers". Second, in levels with reusable numbers, always verify the difference between the sum of non-reusable tiles and the sum of the "goal numbers", as that difference equals the amount you'll have to supply with the re-usable tiles. I know these are plain obvious observations, but they're the kind of thing we tend to forget when we start getting frustrated, and believe me when I say they make a world of difference.
As for the presentation, it's okay. It's nothing extraordinary or flashy, but it does its job well. I particularly love the music, and the graphics are not bad.
All in all, a solid puzzler for people who like to crack their brains a bit.