We were there last week and arrived at HS a little bit after 8 am for a 9 am opening. We were the second family at our tapstile (is that what they are called?). Those with dining reservations were let in to the left, and the word on then street was that they could sign up before their ADR. They let everyone in a few minutes before 9 and people hurried to the JTA sign- up area. We did not run, but walked very quickly, and only had to wait about 5 minutes to sign up. At that point, the CMs were holding the crowd back at various points in the park, so we got in the mob for TSMM. It was a sea of people going to it once they dropped the ropes and I remember thinking that the line would be an hour long with all of those people going there, but somehow it was only a few minutes.
Whether the pre-opening ADR is valuable to you depends upon what your priorities are. It would have been nice to have been signed up for JTA and enjoying breakfast instead of waiting to be let in the park, but I doubt we would have been done in time to catch TSMM without a long line.
BTW, my kids could have signed up for the 10:30 show, but we elected for the afternoon so we could do the other things we had planned without rearranging our schedule.