We've been using these for about a week now and so far so good.
We make them by putting half cooled boiled water in them ready and keeping them in the fridge. Then when we need one we use one of our old avent bottles (sterilised) to mix boiling water with the formula, then pour that into the mam bottle. It sounds complicated but it's much easier than how we made them before, trying to rapid cool bottles with a crying baby isn't fun!
It's funny you mention the flow because Lily seems much better with these teats than with the avent ones!