First of all, I wanted to add that many variables are factored in such as party size, speed of the shift's crew at the restaurant, speed of the eaters before you, what the people before you order, if people have been on time to their ADRs... lots of factors change the outcome but here are our experiences for the 4 we have eaten at since Sept 2007 all party of 2:
50's Prime Time- ADR for 12:10PM, arrived 15 minutes early and were seated at 12:20.
Narcoossee's- ADRs at 7:15PM, 6:30PM and 7:55PM during different trips: at 7:15PM we arrived a few minutes early and had to wait until 7:45 to be seated, 6:30PM we arrived 10 minutes early and were seated right away, 7:55PM arrived 25 minutes early and were seated 10 minutes after arrival.
Le Cellier- ADRs at 12:10PM, 12:15PM, 5:55PM and 6:10PM during different trips: at both 12:10PM and 12:15PM we arrived at noon and waited under 5minutes, 5:55PM we arrived at 5:30PM and were seated right away, 6:10PM we arrived just after 6:00PM and waited 30 minutes.
The Plaza- had ADR for 1:00PM but we were so hungry we we asked to be seated asap when we arrived at 11:40AM, they took us in within a minute.