Does anyone have any experience with the ATMs on Santorini? I have a VISA ATM card partnered with PLUS and INTERLINK. What is the per-transaction fee? I'm trying to decide whether or not to take cash and TC's or join the Techno world and use my ATM for the first time.

Is your card an actual credit card? Or just an ATM/debit card affiliated with the PLUS system.
If just an ATM card, the fee depends on your bank's charges for using out-of-network ATMs. A small number of banks will tack on a % in addition to the VISA/MC 1%. Check the fine print on your bank's ATM cards for additional fees.
If you are using a credit card, you are looking at cash advance fees. Your credit card terms and conditions will detail what the charges are for cash advances. Usually the charge is a % of the amount of your advance, with a minimum charge (these are not small charges). In addition, because this is a loan, you may also be charged a certain % until the "loan" is paid off.
It is better to obtain cash via an ATM/debit card than through credit card cash advances.

I used ATMs on Santorini, and they work just as they do all over Europe and with same fees. No problem finding them on Santorini, at least in Fira and Oia.

I used my atm on Santorini last summer and had no trouble. There's a greek bank (can't remember the name, but they have a location at the Santorini airport) that worked out fine. Nibblette is right about the bank fees. You have to ask your local bank about that. I asked mine and they said the fees would be $1.50 or $2.00. This was fine with me, for the convenience of not carrying a bunch of cash. We took out a couple hundred at a time, and it worked out great. I also agree about the credit card use- don't get cash advances through those. The rates are far worse.