I've been using starhub prepaid sim for a while now and have experienced no problems with it.
You buy it at the Starhub centre in singapura plaza, but a lot of newsagents sell them as well. you can get the recharge cards for them at great deals in lucky plaza ($16 as opposed to $18). That gives you quite a lot of talk time and tons of free sms. If you're like me and live on sms, that recharge card last a while.
right now, they want you to register the sim card. that is simple. there is a booth outside lucky plaza, as well as the starhub centre. you give them the sim card, they ask you for your IC and address (or whatever form of ID). that's it!
to call overseas the rates are listed plainly on their website, however, they do have deals where you can call freely to malaysia, the US, and other places, all you pay for is the talk time. if you really want to call cheaply, then you can get international calling cards that cost about $10 and you can talk for a long long time. just check for one to call thailand if that's where you predominantly call.
just so you know, the prepaid sim card i think costs $28 if i'm not mistaken, it's been a while
enjoy, like i said, i've had no problems with them.