My baby was breastfed for six months until we had to switch to bottle feeding for medical reasons. She did not get on well with either the slow or fast flow teats, but took to the variflow teats very well. They have an X cut in the tip rather than a single hole, allowing the rate of flow to change depending on how hard the baby sucks, which is supposed to be more like breastfeeding, so that may have helped her to make the transition.
My only complaint about this product is that the vents sometimes stick after they've been steam sterilized, so that the teat collapses in on itself as my baby sucks, and she has to stop drinking to allow air back in to the bottle. It is very annoying when it happens and I haven't figured out a way to unstick the vents whilst keeping the teat sterile yet, but it only seems to be a problem for the first few uses of a new teat anyway.