Not a bad game for if you have time to kill and some patience. Throwing the typical order of operations does make this game a tad hard to get the hang of at first, but it does get easier with experience.
However, there is something to be desired as far as user interface is concerned. Some background music would be a nice touch, at least in my opinion.
The game does get repetitive fairly quickly, but then again, that is the nature of mathematics. I would have to say that this is one of those games that is good for a mental mathematics workout, which is good from time to time.
For the most part, the medals are straight forward, except for the "Uncharted Territory" achievement, which is the only one I have not been able to figure out thus far, which is the sole thing keeping me from getting the 36th achievement, and the subsequent Equation Master award. Not having a given goal for it does make it a tad maddening, as it is akin to trying to hit a bulls-eye on a dartboard at night, but such is the nature of the game.
Overall, the game is fairly solid, but seems like a little tweaking would help a lot. It is something different, which I feel is refreshing every now and again.
This game is not for everyone, as each person has their own tastes and preferences and abilities, but it is well worth trying out, again because it is a different type of game, as compared to the mainstream.