I had a better UX around playing music with WinAMP, Microsoft Speech API, and a little program I wrote to glue the two together via Win32 API WM_ messages, in 2007, than I have today with Google Now.

Today the problem is a) the voice recognition part, which happens on-line though it shouldn't (introducing large latency and unnecessary Internet dependency), and b) scriptability, which is something none of the voice assistant vendors want to give to you, for business reasons. A better reality would have voice parsed on-device (possibly with models from on-line service providers), and available as a voice-to-text API that could be easily accessed by local scripts.