I think its useful as a development tool and character generator, but it the biggest shortfall I see is that classes and prestige classes are limited to those found in the core books. It also has no way to handle monster templates (well, you can create a new race to handle this, but the hit dice are going to wrong and you'll have to change them manually).
On the other hand, its easy to create characters and generate a stat block, character monsters are easy to generate, new monsters and races are easy, treasure is easy. Adding custom information varies depending on the type of stuff you're trying to add. So the overall grade I give it is a B. If they add a way to generate new prestige classes, or customize it for a different D20 build, I'll upgrade my evaluation to an A.