ATMs are everywhere, and international withdrawals are not a problem, besides the fee. An ATM will typically provide a much better exchange rate than many money exchangers, especially if you have a card that does not charge a transaction fee for overseas withdrawals. However, note that there are some money exchangers (especially in Bangkok) that have highly competitive rate, even better than ATM.

There is a 220 baht surcharge for using foreign cards in most ATMs, you will mostly be notified about this fee in any ATM which charges it beforehand. Aeon, which charged only 150 baht before, doesn't seem to accept foreign cards anymore.

Most ATMs have a limit of 20 notes, that is 20,000 baht; Bangkok Bank typically dispenses 25 notes at once, and a few other banks including Krungsri, TMB and CIMB may dispense 30 notes.

The more important thing to watch for is that some ATMs (Krungsri, SCB and a few others are known for that) will offer you to exchange your money to Baht for you, charging your card in USD or your home currency. What you will get if you agree is a very lousy rate (-5% if not more from the mid-market level). So, always refuse and choose to be charged in Thai Baht only, not USD or your home currency. Read more on Dynamic Currency Conversion.

Very (very!) remote areas (including smaller islands) do not have banks or ATMs, so entering those places with cash is essential.