Michael Mace:

The most beautiful app is not the one that looks most striking; it’s the one people can actually use. You should design your app to be usable first, and then make it as pretty as you can. The highest form of beauty is functionality.

This pretty much goes for all design, but it’s especially true of mobile design where (a) the usage contexts are so extremely varied, (b) the devices are relatively small, and (c) standard UI and UX practices are still being learned and developed, and perhaps just now are maturing out of their infancy.

Style vs. Substance in Mobile Software