Spacechem Mobile $5.99 -> $2.99 (iPad only)
Plan to download this once i get home to my iPad. Have heard great things about it on PC and hadn't realised that an iOS version existed before today.
It's supposed to be a properly challenging puzzler that forces you to think for yourself. Pocket Gamer's 9/10 review mentions taking upwards of an hour to solve a single puzzle in the later stages, so sounds great for those looking for a slightly deeper experience.