I like the Olfa Deluxe Rotary Cutter as it has an ergononically designed handle & the blade automatically closes when you release your grip - a great safety feature if you happen to drop the cutter. Check this site for a picture. I cut at least 4 layers frequently & seldom use my blade sharpener. I think the blade closing automatically saves it from nicks.
http://www.google.com/products/catal...=com.microsoft:en-us:IE-SearchBox&oe=UTF-8&rlz=1I7ADBF&um=1&ie=UTF-8&cid=6893584314794472125&ei=DD-4TMXoAcH6lwf_4PS5DA&sa=X&oi=product_catalog_result&ct=image&resnum=3&ved=0CDoQ8gIwAg#