I purchased a couple of apps recently based almost entirely on their quality of design. The first was Pushpin for Pinboard, which is a minimalist, fast and beautiful app. I really just wanted to play around with it but I've started to keep a public library of interesting reads on the service.
The second is Waterlogue which applies a pretty convincing watercolour paint job to your photos and has an exceptionally well-designed interface. It's not a killer app but it is entertaining.
This is also my first post written, edited, photographed and published entirely on my iPhone with Squarepace's recently launched Blog app, which goes some way to explain it's brevity. My MacBook is with the Genius guys for a minor repair.