In a search just now, Delta Main r/t from ATL-LAX is $337. Delta Comfort on the same flight is $727. (Yes, it's more than 2x the price.) Obviously Comfort boards earlier, but it's not unreasonable to attribute most of the fare differential to the legroom.
I've never understood why boarding early is considered a premium worth paying for (for normal flights with assigned seating). Planes are never as comfortable as whatever I have access to in the airport, whether that's hanging out in the lounge or sipping a drink at a restaurant, or just stretching my legs walking around the airport.
When I used to have carry-ons, boarding early ensured that I got room in the overhead bins to place the bag. I now use under-seater bags only and pack light so I no longer care when I get to my seat.
Yep. I've taken several flights in the past few months, and _every time_ by the time my boarding group was called, the staff announced that there was no more overhead bin space, and people would have to check bags.
Depends on if you packed wisely (not needing to remove medication or batteries before gate checking). Also depends on how quickly you need to be able to get out of the airport when you arrive.
Lastly, there is always the risk of a lost bag once you no longer have it with you. One fight years ago they forgot to load all of the car seats and gate checked bags that were left at the end of the ramp. We were stuck waiting 90 minutes for the next flight from that destination to arrive since we needed the car seat to drive our child home.
Checked bags are also extra for either seat.