I have one on my mill. It works fine, and has enough keys for all the functions I need.
However, I also have a Genovation control Pad for my router, and like it better. It is cheaper, but more important, the X-keys requires a special driver to be loaded. The driver reads the key code form the keypad and converts it to the string you aet.
The genovation has a download feature- you load the actual key strokes into it, it then sends those, no drivers needed. I always like not having to add special drivers that may or may not be happy with Mach.
http://www.genovation.com/I found my genovation on ebay for about $50.