First thought is the maintainability of the code base. If you are building an iPhone game that will be out of the charts in 3 months and will never be updated,

First thought is the maintainability of the code base. If you are building an iPhone game that will be out of the charts in 3 months and will never be updated, then go ahead with feature code delivery and no tests. If you want your system to still be working in 2 years time and you still be able to add new features to it in 2 years time then you need tests. The more confidence you can get in your code change the faster you can add features. Comprehensive automated, fast running testing gives you this in spades.