That's really very impressive, I can imagine coming back to this as a rapid-sketch tool in a way that I never could with SketchUp.

One suggestion: To aid camera/scene navigation, you might want to consider hotkey + drag to temporarily flip into camera move modes, even when in draw mode; many desktop 3D/CAD tools offer this. I'd opt for ctrl/⌘+drag for rotate, and allow your existing shift+drag for panning to work in draw mode.