I have been interested in the Revolabs x-tag microphone for a while now, but since it costs $250, I expect more from it. There doesn't appear to be any way to turn it on and off, other than on the base station. Perhaps there is a switch on it somewhere. If you are going to wear something on your chest, of course, you are obviously going to pick up sounds other than your voice, which is why they recommend the x-tag only for relatively quiet environments.
I think you are better off with the Amulet for $150, unless you really want to be hands-free and you don't want to wear a headset. My preference, if I was going to pay that much money would be a really comfortable light weight and high quality noise cancelling headset.
Kalle, the link that you posted to nuance doesn't seem to be going to the right place. Can you check it please?
I should warn that there are some bluetooth microphones that have been "approved" for use with Nuance Dragon Naturally speaking, that are not recommended for use with VoxCommando. The reason is that Nuance has a special sound profile that tries to overcome the deficiencies in the bluetooth signal. Because VC uses the Microsoft speech engine we can't do that, but in any case even with Nuance's engine the accuracy is (from what I've read) still not very good.