I have really liked them. They are sharp enough for me and cost $2.00 vs $10 at my LQS.
http://www.harborfreight.com/gsearch?q=carpet+cutters. The blades that come from Olfa and other manufacturers are oily too and if you wash them it could dull them and cause rust. If it bothers you just wipe them down. Even stainless steel rusts. I use the blade as is and have had no problems. The oil is to lessen friction in the cutter. They have to be super sharp to cut carpet. I haven't had a problem yet and even if you get a bad one it is still cheaper to try another.