It will definitely be more expensive to operate than a comparable gasoline model, but I don't think most users would notice it. Even for me; I mow/trim just under 1.5 acres and even though I use my 2-cycle string trimmer 3-4x a month during the mowing season, It's only about 30min of use per month. If one little propane cylinder lasts even close to 2hrs like the manufacturer suggests, one or two cylinders would carry me through the mowing season. No need to worry about stale gasoline/premix sitting around, carbs and sparkplugs fouling, gasoline leakage, etc.. I think it's a decent approach for the average homeowner.
Joel