Singapore experience was not great
Both M1 and SingTel are listed as partners in Singapore.
My phone roamed onto M1 by default. Text messaging flat-out did not work for me on M1, period.
Manually selecting SingTel was spotty at best. It took quote a while to get registered successfully on the network, and SMS was not reliable (about 25% of the time I couldn't send texts).
Other places I've used this service include Taiwan, Thailand, and Indonesia. Didn't have any such problems in any of those places -- was able to get registered on a network immediately, with reliable texting.