On Windows, I use eshell a lot, because I use Emacs anyway, and it provides a more familiar shell experience while also working smoothly with Windows instead of seeing the whole system through a Unix-lens as Cygwin does (i.e. Windows paths work, you can say //myhost/myshare/myfolder/myfile.txt to directly open a file on a network share, etc.)

On Un*x systems, I use it a fair bit, too, because its integration with emacs is more comfortable than running a regular shell from within emacs. Being able to call elisp functions from within the shell is something I do not do often, but it is very convenient have anyway.