The exchange was about 500 colones for every 1 US dollar – so 1 mil was $2, 2 mil $4, 5 mil $10, and 10 mil $20 – my budget never saw higher than that.
The value was about the same as in the US, though. Which was surprising – I spent way more than I had intended.
The size of US tourism, combined with the dollar being more stable, meant that most places – taxi drivers, bus stations, vendors, hostels, restaurants – took American dollars. I’m not sure if that is technically allowed, but it is widely practiced.