Ok. I have been using an acrylic case now for 3 years.
Benefits: No vibration. It is plastic. It doesnt resonate anything. Hd is mounted directly to the plastic housing, no audible seeks on a 7200 enterprise drive. every other case i had, it was difficult to get the hd from making vibrating noises and transmitting sound.
also: no rattling
also: you can see when it is getting dirty and know where to clean
People buy steel on these forums to either trap sound or something, I am not sure. I dont really see it. They make musical instruments out of sheet metal, not plastic. Only problem is that working on hd cages is like MAJOR issue. You sorta have to take apart 70% of the case. However, I only use 1 big hd and just use my delete key on useless crap. Once it is in, stays in for apparently 3 years.
people worry about fingerprints. my advice: Use soap in the shower, or dont work on your case while eating Kentucky Fried chicken. Otherwise, it realy doesnt show fingerprints as easily as claimed.
benefit: all those funky lights make it go psycho
*Rave music pumping*