I came to use it because I wanted to simply show different modules to regged, or non regged users.
(something Joomla should have built in...IMO)
This accomplishes that...easy,
When I installed and fully began to see this modules full parameters, I was like whoa dude...I can implement this on other sites for a multitude of reasons. Awesome!
Drawbacks: not many! I had some trouble setting up the PHP "recipies" to fit the enviroment I was working in, but through some trial and error I worked it out.
I also had some small spacing issues where the specified modules displayed, but that was in my template .css and that was an easy fix.
If you a true "Joomla head" and build alot of sites this is THE trick to have up your sleeve, it's very flexible, powerful and FREE!