I read some of the bad reviews on these bottles and never had any problems with leaking liquids.
It is true that the few few sucks they seem to take in a lot of air, but if you burp regularly during feeding (which, in my opinion, you should do anyway) then it doesn't affect them.
I exclusively breastfed my baby for about 2 months, slowly introducing bottles in the last month before I had to return to work full time. He had the hardest time taking ANY bottle. He'd fuss, cry, and push it away until I gave in and just nursed.
When it came time to go to work, he was freaking out with his grandparents. He refused to eat all day long with them and just cried and cried until I got off and picked him up. I was devastated, and it almost drew me to quit my job so that I could be with him. The thought of him wailing all day long broke my heart.
So, desperate, I went to the store and bought every brand of bottle they offered! Dr Browns, Avent, Playtex Drop Ins (with several different nipples), Training Sippy Cups, Adiri, Nubi, Prince Lionheart and of course First Years Breastflow. One weekend I tried every single one. When I gave him the breastflow, he latched on immediately and GULPED in a way I've only seen him do while he's nursing! He even fell asleep in my arms after the feeding!
Truely, to this day, he will only eat from these bottles and I'm so comfortable going to work now. These bottles offered me a relief from my anxiety, and I thank the person who designed them with all of my heart.
Reading the other reviews, these bottles may not be good for a colicky baby - however, if your baby is like mine and has a major aversion to the bottle after exclusively nursing - it's perfect :)