I have used the M1 card and my friend uses SingTel.

Both were easy to find and easy to recharge. I have been back 4 times in the last year and I just put the card back in and recharge a little to extend my time a half year.

One unusual thing is that on M1 you receive a text message after each call telling you your balance. Really more than I need to know, since you can always call.

Also, incoming calls are charged like in the US. So it is a local call to your mobile from a landline, not a premium amount as in most GSM countries. The rate is the same for incoming and outgoing.

There is also a variety of ways to make international calls on M1, you might want to check their website for details as they vary by time of day and the service you select when you dial (access code)