I've been developing sys admin scripts and tools on cygwin for a little over two years now (primarily because my company's standard workstation is Win2K -> WinXP) and there is no way I would keep my sanity in tact without it! I just recently started using the XFree86 tools and wished I had started using them earlier!

I can work on my web server reports off site without any risk of harming mission critical servers or data...this list goes on...

-hieronymus