i just bought the peg perego convertible seat for grandma's car. seriously, I am in love with this seat. I also bought the foonf awhile back but have yet to test it. I needed something that is easy to use for grandma and breathable, and comfortable for my little girl. and I have not regrets. I will test the foonf this week and if it doesn't meet my expectations then I will definitely get another peg perego for my car.
I recently tried the nextfit and the fabric is breathable, however, adjusting/tightening the harness was terrible. it was so difficult that even my strong husband complained about it. there was absolutely no way for grandma to ever get the harness to fit properly on our little girl, thus rendering that carseat useless. I really wanted to love the nextfit because cushion seem so heavenly and everyone raved about it. in the end I returned it after giving it a try for a few weeks. despite what we read online from all these stellar reviews, my entire family hated the nextfit (completely overrated) and decided to try the peg.
the peg is in my car but soon will be transferred to grandma's car (we finally found an easy to use convertible carseat for her!) my girl sweats little in the peg perego despite the summer heat, and tightening the harness is a dream...it is even easier than using our infant seat. however, installing the peg is a pain with a seatbelt (we want to have her sit rearfacing in the middle seat). I may have to use the latch system in grandma's car and give up keeping my baby in the middle.
so really, look into the peg as a choice. the foonf is also breathable, but I am a bit intimidated when it comes to installing the foonf...too many parts, but that is my next goal.
for the price and quality, peg perego is the way to go.