I have an Ergo, but I don't have the newborn insert. Does the insert work? Is it worth buying one? Or do babies get too hot in Singapore?

I'd say it depends on how old your child is and whether he/she is comfortable with a carrier in the first place. My son belongs to the type of children who do not like carriers, wraps, slings etc - so he was always carried on our arms until he was about 7-8 months old before we started using a stroller! For the issue on newborn inserts, it's not a matter of how hot it gets - but the child's safety that matters more. You'd have to see what age, height and weight the carrier is suitable for - and when they'll need/can do without the insert. If your child is below any of the required criteria, please do not put your child inside the carrier (or whatever it is!).