My commute is 40 mi rt w/one 18% .4 mi hill. Depending, I'll do switchbacks or grunt my way up straight on 2-3 times per week. In my area I've got rides laid out from 20 to 120 miles that I do on singlespeed. There're several 5-14% grades within those parameters. This year a guy finished the Pittsburgh Dirty Dozen ( http://www.dannychew.com) on a fixie w/39x20 gearing. The easiest hill is a 24% grade grade w/t toughest being a thigh burning 38% on cobblestone. The rest fall somewhere in between.

IGH would be optimal, of course, but going to fx or ss is doable. One just has to select the right gearing. I use 46x16, but will go to a 48x15 fx asa I get the $$ for the components(wheelset, bb, cranks, etc) for my old Ross frameset. When I finally made the jump it took me about a week to adjust. My overall strength on hills improved as well as my technical skillset.