I agree about National, and especially Emerald Aisle. The problem is that if you want anything except the basic car (e.g., the child seat or an upgrade) you have to go to the counter. NJDavid may have had the same problems had he been using Emerald Aisle. I've tried to use upgrades when I return the car and been forced to the counter then - which can be more frustrating than going to the counter when you arrive. Especially if you're in a hurry to catch your outgoing flight.
My guess is that at least Hertz and maybe Avis would have the same problems if you tried to use #1 or the Avis equivalent and needed something special.