Just to add my 2 cents, I've tried both StructureMap and Unity. I found StructureMap to be poorly/misguidingly documented, a pain in the butt to configure, and clunky to use. Likewise, it doesn't seem to support scenarios like constructor argument overrides at resolution time, which was a key usage point for me. So I dropped it and went with Unity, and had it doing what I wanted in about 20 minutes.
