I got the Rokblokz fender extensions from rokblokz.com. The install took a little time, but it was well worth it. They fit great and keep the mud off the side of my machine very well. I touched up the stainless steel mounting screws that go around the fender with flat black paint using a cotton swap/q-tip so they blend in better and don't stand out so much.
They do cut down on the dust level getting inside the cab. I ride a lot of dirt trails, so I still get a layer of dust on everything, but they for sure help with keeping the dust from swirling around inside.
SD will be our home someday! Been riding their twice and we planning a 2 week trip next summer. I will say both times we have ridden the Black Hills we met up with some locals with SXS's and followed them around. I was impressed with their riding skills.