We were there 3 days starting on Halloween night. We got in during the 3 hours before the party and I'll tell you, that whole weekend the crowds were lower than I ever expected. On Halloween, we walked on most rides or never had a longer than 15 minute wait. And that was after my daughter finished a couple treat trails and watched the parade. The Saturday and Sunday there it was about average crowds... nothing crazy. We got to almost everything we wanted to. Again, the longest we waited in line was maybe 45 minutes for Haunted Mansion. Thrill rides we did single rider or fast pass. We got a fast pass for Elsa and Anna M&G about 90 mins after opening with no problem (no line to get too)... unheard of most of the time on a weekend.
We will continue to go on Halloween based on our experience this year.