Ability to dial in levels of detail, ease of use

Crazy bump is very simple and quick. UI, ease of use and speed.

Supports great features, such as the addition of grunge maps. Being open source, along with the price of nothing makes this. It is easy to use and does what it says on the box,from memory the Nvidia one has the advantage of working inside Photoshop , there's also Xnormal if you want to create normal and height maps