I'm the type that likes to get to places early alot of the time. For example, I like to get to work half hour or 45 minutes early so I can sit in the breakroom and have my coffee and visit a little. So alot of times, we'll get to the restaraunt early, either for that reason, or because we've pretty much done everything we want to do in the parks, or everything we have time to do without encroaching upon our ADR time by standing in line plus the amount of time to do the attraction. A good portion of the time when we get there early and let them know we're there, we end up getting seated early...as in 10, 20, 30 minutes early.
We had an ADR for 50s Prime Time back in April. I can't remember exactly what time it was for, but we got there a half our or 45 minutes early (I can't remember exactly how early), and they came and got us in about 10 minutes. We don't usually ask to be seated early. I just say, "ya we have an (insert time here) ADR, and I know we're a little early but we thought we'd just check in and let you know we're here".