Originally Posted by snrusnak
Underdrive pulleys do help. They do not increase the power output of the engine, but free up some power by taking away some of the parasitic loss on the engine accessories. It takes some power to turn the accessories, so if you use an underdrive pulley you will pick up a couple hp. They are usually lighter as well which helps this same concept. Some people that use them and have large electrical demands (audio or video...) also need an overdrive alternator pulley to keep up with the electrical demand. Most companies that sell underdrive crank pulleys sell matching overdrive alternator pulleys. Be careful and do some research on the company, some people have run no name underdrive pulleys/harmonic balancers and ruin their engines. The harmonic balancer/dampener is a vital part of the engine, it soaks up harsh imbalances(harmonics) that would otherwise destroy the internals if it weren't there. I've seen a guy take all accessories off his engine including the crank pulley/balancer, and didn't even make it one pass down the track before his engine blew...
That price seems a little cheap from other prices I've seen, but if you find others using the same product from the same company for some period of time without any damage, it's probably fine. One of the biggest names in quality dampeners is ATI just FYI maybe you could compare...
The original motoblue was blowing a lot of hemis back when they first came out. Not sure if the new one's are any good but I personally am leery of motoblue after what happened with their original attempt at an underdrive.