I recently bought the MXL AC404 USB conference microphone. It is really incredible. I can speak *very softly* from 25 feet away (as far as I could get) and it picks up with ~94 percent accuracy my commands (quiet room of course). If I have music on, I need to speak at normal volume. I was in my kitchen about 20 feet away, behind a wall, with the sink on and it still accurately accepted commands. I have the mic on the entertainment center behind my tv, which is probably not the best spot, but it still works great.
My current setup makes for a good mix of remote and voice commands. Sometimes it's easier to navigate with a remote, sometimes with voice commands. For example, I have a USB uirt, which can send IR to all of my devices. So I can turn on components (tv, stereo) with voice commands from virtually anywhere. I can select different tv inputs and it jumps right there. I give voice commands to get to where I want within an application and then use the remote if I want to navigate or browse items.
In addition to voice commands for listen/ignore phrases, I have the red, yellow, and green buttons on my harmony remote set to stop, standby, and on modes in VC. I use the blue button to mute and listen, for when I have music turned way up.
The only bad experience is that it has started listening while i was watching a movie a couple times (accidentally 'heard' my listen phrase). But I think that is pretty inevitable with a whole room mic and a very loud movie. Now, when I start watching a movie I just press the red 'listening off' button on my remote. But I might setup a way to have vox or eventghost detect movie playback to automate this step. For some reason, watching live tv in WMC disables VC, which is nice. But becomes 'aware' at the WMC main menu so I can command it to 'go back to XBMC'.
If you are looking for a mic that can pick up voice commands from anywhere in a large room, this is the one. But understand the limitations of a very powerful mic (it hears everything). You just need to get creative with eventghost and the groups and commands in VC, but overall I'm still very impressed.