Before I get into the review, just to make sure prospective players get this warning: Dont get this game if you only have an iPhone. The screen size really makes the controls difficult, but it should be fine on the iPads.
Now thats out of the way...
I greatly enjoy the physics engine that Grey Cubes uses, the way blocks bounce and react to being hit. The graphics are pleasant but minimalistic and for the most part I enjoy the level design.
But it also has a lot of annoying quirks. The biggest is the limit on how many balls you get in a stretch. You start the game with five, and its almost impossible to replenish them without spending money or spending points off your score (which, naturally, is finite). If you run out, you go back to the main menu and restart the level with just that last ball remaining. Over and over. In this day and age, having this limited number of lives just stinks of pointless retention of the arcade "feed more quarters" mindset and is clearly intended to annoy people into spending money on in-app purchases.
Second, the aforementioned cash store. The in-app purchases dont do anything like add new functionality, levels, or gameplay. They just incrementally lower the difficulty by cutting out debuffs and increasing the rate of certain powerups (which arent even that useful 90% of the time). Spending money should improve the game, not make it less bad. Pro-tip for game developers: if you want to make more money off your game, maybe just dont let Starbucks give it away for free.
There are other little annoyances, but these two are the biggest that should factor into your purchase. (Honestly, dropping the limited balls would be enough for me to round this up to five stars) Im enjoying it, but that feels like its despite the devs choices and not because of them.