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.
Blenders interface is built with power users in mind. They've made some progress in reducing how steep the learning curve but they still have a ways to go.
Your right about how clean/responsive and fast it is. They have justifiable pride in how many features they include in such a small binary. When I was hacking on it Blender weighed in at under 10 megabytes with features other apps needed 100s of megabytes to provide. I think they still come in under 20 but I haven't checked in a while.
Have you tried the new Blender 2.5 (now in alpha)? The interface is a major improvement... It's much easier to find things in menus and figure out what various options do. And you can edit the camera movement keys to match other 3D applications. There is still some weird modality, but I think this version's interface is becoming normal enough to see some significant adoption once it gets out of beta.
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.