When I became pregnant I wanted to find a bottle that would reduce gas in my future newborn baby. Then all the media started talking about the BPA chemicals in baby bottles. I researched and found that the playtex drop ins were not suppost to have that chemical in them and they were designed to reduce gas and colic in babies.
These particular bottles have plastic liners that drop in them that collapse when the baby sucks the bottle. This set came with 2 4oz bottles and 2 8oz bottles. It included the slow natural flow silicone nipples and also included breast milk storage components.
The good point of these bottles are that they are very easy to clean. Throw the liner away and toss the rim, nipple, cap and bottle in the dishwater.
They do help with some gas and colic. However when my baby was born he had reflux. So I did end up switching to a different kind of bottle due to the reflux.
A downside to these bottles is that you have to keep purchasing the liners for the bottles which can become expensive.