I don't keep many games on my iPhone, but I am fond of the pick-up-and-play variety -- the type of game you can jump into at any time and enjoy a few rounds. Super Squares (free with in-app purchases) is a good example of this type of game, in that it's effortless to learn, challenging and fun.
The game looks great and the idea is simple: draw lines between like-colored squares, capturing "stars" as you go. You start with a small grid and a pair of squares. There are some stars on the grid, too. Your job is to trace a line from one square to the other, drawing over the stars as you go. You can't cross the line at any point, nor can you double back. It's easy enough to begin, but soon gets quite tough.
Soon multiple pairs of squares (pink, green and blue, for instance) and Mario-syle "blocks" that impede progress are introduced. The blocks can be destroyed with bombs (more on that later) and you have five opportunities to re-do the lot in one fell swoop (you can "erase" a path by tracing back over it for free). This is where we get into trouble.
As you progress, the puzzles get tougher. More colors, more blocks, more opportunities for getting stuck. Eventually, you can't progress without bombs to destroy the blocks, and the only way to get bombs is to buy them through in-app purchase. Once I hit level 24 (of the initial 300 free levels), I had exhausted my initial supply of bombs and couldn't progress unless I bought more at US$0.99 for a set of 10.
I'm fine with in-app purchases, but not when they hinder gameplay. In the case of Super Squares, I can't progress unless I buy something. If there was an option to laboriously grind away for free, I'd opt for that. Heck, I'd probably even make the IAP just to avoid the grinding. But taking that option away and requiring me to make an IAP leaves a bad taste in my mouth.
Super Squares is a fun, simple, challenging and attractive game. Unfortunately, it's hindered by an overly aggressive IAP scheme.