Cygwin is impressively complete - if you haven't checked in on it recently, there are over 5000 packages in the official repo (that's more than Arch Linux's official repos), and adding the http://cygwinports.org/ repo gives you full KDE, GNOME 3 and Xfce on windows (although KDE has a first-party port).
What this tool offers over Cygwin is the ability to run unmodified linux binary-only software for which you do not have the source code. For me there's not very much of that although i release some at $DAYJOB.
What this tool offers over Cygwin is the ability to run unmodified linux binary-only software for which you do not have the source code. For me there's not very much of that although i release some at $DAYJOB.