I'm not a 3d designer, developer, nor any standard general user of Blender really, but from my limited trials of usage it's almost certainly the most well made, polished piece of open source software I've ever used.

It's UI paradigm in particular is at first somewhat overwhelming and unintuitive, and then suddenly with the tiniest amount of instruction or direction perfectly understandable and infinitely flexible (once you "get" how it works you can imagine them adding any feature and it being just immediately, simply usable)