I had my under drive pulley installed the other day, and I can feel a noticeable difference in power. I think it's a great mod if people don't want to do aftermarket exhaust and intake and want a little boost in power. Or, if you already have those mods and you're running out of stuff to do, don't over look the pulley.
I'm attaching a link to Wstar's journal. He did his own install and included some pictures.
wstar's Journal