Matt,
I briefly looked at the CN100 USB motion controller. I didn't find enough data from folks who were using it to make me choose it. CNC4PC has good products so it is probably exactly what they say it is. You should try one and let us know.
I chose an Ethernet controller because I felt it did more and suited my needs better. Fairly soon there will be a bunch more choices on the market. Hopefully folks will share what they learn with us.
One other reason I chose to get the Smoothstepper directly from the manufacturer instead of CNC4PC was the shipping costs was much less.
Bill