it's been decades, but my experience was that places with commission would only be better if you change large sums of money, which, being a student on a budget, i never did. and if the commission is a percentage, then how is that different from a bad exchange rate? it's simply a matter of math.

i remember annoying my hosts once because i insisted on checking 3 or 4 places to compare rates and make sure i didn't get ripped off.