I have three Olfas - one of each size. I use the 60 most often and they work great.
However - that being said - they are uncomfortable to hold I think. I was at a quilting Expo - and saw the Martelli ergo 60mm and tried it out - It was fantastic and fit the curves of the hand perfectly. When I get the chance I will order it from Hittle. Here is the Hittle link
http://jhittlesewing.funoverload.com/sewing/catalog.php
Here is the Martelli Link as well
http://www.martellinotions.com/products.asp