The previous Enlightenment, now known as E16 Desktop was designed to make use of idle cpu power, in order to create a special and great looking Desktop.
Other than a few annoying multiplying epplets, it was great, fully customizable, fast and didn't get in the way.

E17 has been coming for many years. Unfortunately development was put on hold for a long (very long) time.
The good thing is that it's currently back under "active" development, and finally, about to be released !

Why would I want to use enlightenment ?

Simple, because you can.
One of the best things about linux, which at the same time is one of the worst things about linux is the many flavours.

The best I can give you is why I chose enlightenment over the other desktop environments.
The reason is quite simple.

It's fast, responsive, and easy to manipulate many different programs / windows at the same time.
The balance between speed, usability and user-friendlyness fits me exactly.

Everyone uses their computer in their own way. Maybe enlightenment will fit your preferences like it did mine.

Are there any other E17 distros?

Plenty :)