I know for me when using Siri, I am disappointed 80% of the time. Siri fails actually at least 80% of the time when I use it, so I really cannot use it, because it's too annoying to keep trying until Siri gets it right. Much easier to type than to try and talk to Siri.
Google glass is going to have significant challenges with the voice commands and early adopters will be willing to go through the pain of asking it several times, but the mass market will quickly get frustrated if the voice commands do not work 90+% of the time.
Maybe partially but the real success lies in designing something like this that can be worn without the user looking like a huge nerd. Otherwise it will never hit the mainstream. I mean, I even know many tech early adopters who think its a cool experience but would never use it in public the way it looks now.