From a user perspective, Blender seems to have cleaner code behind it than most of the commercial 3D apps; it starts faster, it crashes less, and I haven't encountered bugs in functionality.

As for the interface - I find it dire for detailed editing tasks(I opt to use Wings for modelling) but not too bad for managing the overall scene, applying materials, lighting, compositing, etc. I speak as someone who's invested a bare minimum of time into properly learning to use it.