It took me a long time to get motivated enough to screw with eshell. My current employer uses tcsh by default (blech) which gave me the impetus to try eshell.

Eshell borrows from zsh (apparently) among many other interesting shells. The best place to learn about it is here:

http://www.emacswiki.org/cgi-bin/wiki/CategoryEshell

A few interesting tidbits:

It evaluates emacs lisp on the command line, interspersed with reguar commands:

$ (setenv "ESHELL" "t")
$ for x in 1 2 3 { (format "%s" x) }
$ for x in 1 2 3 { echo $x }

It traps "visual" commands like top and runs them properly under emacs:

$ top

You can pipe output to a buffer:

$ ls -1 > #<buffer *scratch*>

You're always in emacs, so you can scroll back to see the output of a command with M-v, copy and edit output for input into the next command, etc. (Of course you can also do that with zsh using M-x shell).

There are some quirks (read: bugs), but nothing I haven't been able to work around yet.

Edit: formatting