I have the gel nimbus 17 and love them.  Read some great reviews on the 18 and rolled the dice and ordered them.  Hated them from the minute I put them on.  They felt like dress shoes; clunky, hard, stiff.  I get plantar fasciitis often, I was afraid to even attempt to run in these. I immediately sent them back for a refund and ordered Brooks Glycerin 15.  We'll see how that goes.