I commuted all of last winter on a five dollar garage sale Peugeot (hence my handle) that I cobbled together myself, redished the rear wheel, etc. I eventually stripped the cog and am just now getting around to getting it back on the road & have been riding geared bikes all summer. I also have another fixed gear based on a too small for me frame that I've outfitted with baskets and use as a neighborhood grocery getter. I also have a third project in the works using an older Fuji.
Anyway my commute was pretty short, just a couple of relatively flat miles either way, but I found the fixed setup worked quite well in all except the most extreme (i.e. fresh heavy snow) conditions. I typically carry a change of clothes, lunch and a lock, although just a couple weeks ago I stuffed 14 ears of sweetcorn from the farmers' market into my bag!
I'd concur with previous posters that it's not any more difficult than riding geared.