one person's experience!
I tried it at an AES show, which ought to be a pretty good place to test something like this, and was very disappointed. Not only did it do very little in terms of isolation, but it made the microphone sound bad, or at least not as good as it could.
I've used "shields" in teh past - one built from a music stand and Sonex, one commercial unit. For straight up voice-over work they both work very well (although the commercial one looks a lot nicer!!!!). For a solo vocalist they work well more often than not.
For almost every other application I'd rather a little bleed, they really do change the way a microphone reacts to the room - which is the intention, but for me in a way that is not always positive.