I was using the infant insert at first but found it kind of awkward and unnecessary and stopped at about 6 weeks. The insert also lifted him up higher and made it harder to nurse him. I still have his legs frogged, though.
To nurse him I have to have the Ergo pretty low on my hips and off-center a bit (to whatever side I'm nursing on). Even with those adjustments, his head is still pretty high up vs. my chest. I use my hands to lift up my boob and position it into his mouth and it is fairly hands free at that point although sometimes I have to reposition. If I have a hand free I'll sometimes use it to support my breast. He is usually more upset from being in the car seat than hungry so he often will just nurse on and off for 5 or 10 minutes and then fall asleep (at which point it doesn't matter if my breast falls out of his mouth.) Just in time for me to be done dropping off or picking up his sister and then he has to go back in the car seat where he wakes up and starts screaming again, poor guy.