Ask HN: Fast Action-oriented Launch or Slow Planned Out Launch

37 Signals' book points out that it's best to create some white-board mock-ups and get the site going.

On the otherhand, other programmers/designers (particularly web-design firms) take an extremely long time to (i) mockup up the site, (ii) UML Use Cases, (iii) document the site, (iv) design, etc.

In your experience, what's worked best for you?