I've been running FlashCut for 5+ years. I started with version 1.2 on Windows 98 running a ShopTask mill/lathe. I'm now running version 2.1.4 on XP. It controls a plasma table, ShopTask mill, and currently my Bridgeport Series 1 conversion. Solid as a rock. I only have experience with their steppers though.
Their knee mill conversion uses Elrod Machine components. Which are supposed to be one of the best conversions.
My two cents.