When we tried to go early to our ADR at Sci-Fi, we were about 1 hr ahead of our ADR. They sat ADR's who arrived after us (same party size) before us, if their ADR times were before ours. They told us they were going to do that, though. They explained that if another party our size ran a bit late for their ADR, we might get sat there but since their ADRs were all assigned, they didn't have much flexibility to bump us ahead on the list.
I was not too surprised, it was pretty much like many dr's offices...they'll take you a little early if the current appointment time's party is not yet there.
From the other replies on this thread so far, sounds like it depends. I would not plan on it, especially since ADRs are so booked up now. Like, if your ADR is for 7:15 and you get there at 6pm, and the 6:10 party gets there at 6:01, they SHOULD be seated before you if the table size is appropriate.