We started using level three at 7 months with our second and at 8 months with our first. We never pushed a faster nipple because they were perfectly content with level 2. The only reason we changed to level three was to make the transition to sippy cups easier.
These nipples are SUPER durable. Both my kids have used it. They go into the dishwasher all the time and have held up for 2 years (shocker) and counting.
We have used these nipples with the medela bottles and medela collars with no leaks. Of course, we mostly use these with the Dr. Brown bottles. We have the regular and the options and this nipple can be used for both
Breastfeeding: I mostly pump, but will breastfeed my child occasionally at night. With my youngest at a young age she seemed to switch really easily between nursing and bottle. At 7 months she seems to be more picky and favor the bottle.
Rice cereal: We have used level three with rice cereal. If it is diluted enough it will work with this nipple. This is nice because it is slightly heavier and many say it helps them sleep better at night. We tried rice cereal with this nipple at 5 months. In my opinion, it's nice to feed rice cereal to a 5 month old in a simple and quick way, my kids did not sleep much better because there was rice in the milk.
Overall: Great transition nipple. We have used level three the longest. From 7 months to 14 months. They are super durable and my kids love them.