It depends. An ADR is NOT a reservation. It just means that you will get the next available table that fits your party size that becomes available AFTER the time of your ADR. So, even if you show up AT your ADR time, you may still have a wait, since you have to wait for a table to become available. Basically, it just means that you are "next in line for a table at that particular time".
If you show up early, and they happen to have room for you, and NO ONE else is there with an earlier ADR time than you already waiting, then yes, they will seat you. Otherwise, you could end up waiting a lot longer.
Really, there is not much benefit to showing up early, unless the restaurant has a bar and you can sit and have drinks while you wait.