Personally, I don't use Windows and I know Linux and LinuxCNC fairly well. Generally, it's a hard sell for anyone that doesn't use Linux, but some points come to mind;
I can freely load and update LInux on any PC without having to worry about license and hardware lockout issues.
LinuxCNC is open-source so if I need to, I can study the source and fix or update the software to my particular needs (or hire a programmer to do it)
LinuxCNC was not created to to produce a profit, so there is no feature throttling. If a feature is important enough it will get implemented fairly quickly
Features will not be restricted to what a vendor happens to sell
LinuxCNC has been used on a wide variety machines from toys to industrial CNC's:
https://www.youtube.com/watch?v=mxxdq6y8z8M
https://www.youtube.com/watch?v=AoOWi1r6-Eo
EMCRepRap - RepRapWiki
3D Printing: MachineKit: LinuxCNC ready-to-go f... | element14
LinuxCNC Documentation Wiki: Videos
If one just wants to buy a machine that is nearly complete and just have it (mostly) work, Mach3 on Windows might be the way to go.
If one has a decent background in electronics and software and is willing to learn (or hire someone) LinuxCNC can be very attractive for its freedom, cost and access "under to hood".
There is a version 2.5 live CD available (boots and runs without affecting your hard drive, or if you want, installs everything):
http://www.linuxcnc.org/index.php/download
There is a 2.6 version that comes as a USB thumb drive but I am not sure if it is a live package. I have had very good luck with old Core 2 Duo Dell and HP PC's, but some PCs don't work with Linux well at all.