Personally, i have use Mach2, Mach3, Grex, Smoothstepper, EMC, Centroid and Camsoft.....
We have spend many $$$$!!
After 4 years of testing and reliability issues, we switched all our production of machine on Wincnc(
www.wincnc.net) wich is for us the best of all, good price, good reliability, open architecture so you can built your own macros, screen, tool change etc....It come with a good manual and of course, excellent support!!!
www.ebfcnc.com