Yes very. Most currency is easy to change and we used ATM's a lot too. Only time it got a bit weird was in Colombo when someone said "follow me ..." and we ended up in a tiny room at the end of an alley surrounded by strange men. So left. Fast.
I'd suggest you get the iPhone app from http://www.xe.com as this is fantastic for calculating the costs of things too. over 6 years ago
Stay away from touts asking to exchange your money from the streets. If you have to go to a bank (and I think they require your passport) and exchange your money.
ATMs are reliable and we had no problems so far. It's our 5th week here now in Sri Lanka .. 5 more to go. Check out our blog: http://srilanka.for91days.com for some great tips. about 6 years ago