I have been actively training and running marathons in the Kayano for several years.  I had read the reviews about the Kayano 18 being structured differently so I continued to purchase the 17 until I couldn't find it in my size anymore.  I finally just purchased the 18 and I can tell that it fits slightly differently than the 17, but at the same time I can't pinpoint what's different.  It still feels like a great shoe.  Perhaps some people were right about there being something different with the front width area, but it's not enough of a difference for me to switch shoe lines completely.