When I think of iOS time killers, I think of games exactly like PipeRoll. It's fun and easy, and it reminds me of games I've played before.
In PipeRoll, the goal is to turn series of pipes so that they connect, and water can flow into the house's plumbing. The pipe turning element is a mini-game favourite. You know the drill. That's...not a bad thing.
In fact, I'd argue that slightly unoriginal iOS games are the best time killers, because there's nothing to learn. Going in, you know what you need to do. The game just needs to be original (or enjoyable enough) to make players overlook the recycled mechanic. And PipeRoll does just that.
There are various versions of PipeRoll, including oil-based PipeRoll Oil, retro-flavored PipeRoll Retro, and the historical PipeRoll 2. I played the free PipeRoll, which seemed to scratch my itch.
PipeRoll [Free, iTunes]