We've always done late seating before our DD was born. Now we do main seating mainly to keep in line with her schedule. Her witching hour begins around 7:30pm!
Our one big complaint about late dining with Disney is the lack of availability of good food when we get back on the ship after port. If we aren't eating dinner until after 8pm, we'd like something around 3pm or so and all that was available on our last two cruises is the "theme park food" on Deck 9. I really wish Topsiders had longer hours! On our 14-day cruise, Pluto's and Pinocchio's got really old.
The thing I really like about having early dining is that I don't feel like I have to rush out to make the late night shows. When we had late dining, we always got out too late to get a good seat for the shows in Beat Street. I guess we're too chatty with our tablemates!
I think generally families with young children traditionally take the earlier seating. I thought I read a while back that Disney tends to put those families in the APLAPLA rotation with the extra seating at Animator's Palate and families with no kids in the LAPLAPL rotation to have an extra night at Lumiere's. We like to try to request the rotation that will give us Parrot Cay on tropical/pirate night and Lumiere's on formal night. I thought that was APLAPLA when we sailed the western three years ago.
-steve