This topic just came up in a thread and some people seem to be a bit stuck in the past on it. The findings in this article are, well, pleasing. At least, I was happy to know I can get the same (often faster) peformance as SCSI with a much cheaper Raptor...and thats not even putting two in raid!
http://www.pugetsystems.com/articles.php?id=19
http://www.pugetsystems.com/articles.php?id=19