We use Adiri bottles and are now in the 6mo+ ones. He was getting frustrated at the younger ones, I could tell. I've always had a pretty fast letdown so I watched it pretty closely.
We switched to level 2 around 3 months and i just now bought level 3 (almost 8 months). He seems to get frustrated when drinking a bottle so I'll probably try the level 3 tomorrow and see how he does.
Re: BFers - have you changed the bottle nipple size yet?
I was told not to switch nipples - but this makes a lot of sense.
We have not had to switch, but we use the Nuby bottles with mulit-flow nipples.
Edited to fix spelling.