Charles,
I have one. I agree with Brice. As he said in his review, "... this is not a 'knock your socks off' improvement, I do however find it to be a worth while product."
It's purpose is to improve dust collection. Overall, it improves dust collection slightly. However, it works better when you are cutting in marginal conditions like close to the edge of a board.
It has a couple of slight downsides. First, if you want to do bevel cuts, you'll need to remove it. Second, to use it, you'll need to remove the outside edge green splinter guard. For me, the first is a minor hassle because I rarely do bevel cuts. The second is more problematic for me because I like to keep my cuts as splinter free as possible.
Since I'm more interested in having the splinter guard in place, I don't use it most of the time. However... It only takes a minute to remove the splinter guard and install the dust attachment. So it's worthwhile to have it when I want the absolute best dust collection.
Regards,
Dan.