I used an ATM machine at the Prague airport last November. The machine did not charge a fee for using my ATM card (I haven't encountered ATM machines in Europe that do charge a fee). The bank that issued my ATM card charged me $2.00.
I withdrew 10,000 crowns, which was well below the $500 daily limit on my ATM card. Note that there's an approximate 1% surcharge on the exchange rate (I believe that Visa/Mastercard branded ATM cards impose this fee; I am going to experiment with an HSBC ATM card with no Visa/Mastercard logo on my next trip). This is a slight premium over the wholesale foreign exchange rate and is likely to be the cheapest way to change money.
There are ongoing scams in Prague involving individuals offering to change money with tourists. These should be avoided.
Have fun in Prague and don't drink too much Pilsner!
11/18/2004 ATM ATM MAC KB ATM LETISTE PRAHA 6 420.43