I'm writing a 3D game using agile right now. I'm doing it part time. I started last night. By the middle of next week I'll have an iteration ready for play testing, including a user guide.

Another iteration a week later, another a week after that. Four or five iterations and I'll be done.

And yes, you're absolutely right that unit testing will not catch all bugs. Automated unit testing suppliments human testing. It catches lots of bugs humans miss and humans catch lots of bugs automated testing misses.