On our last WDW trip, we had 5 pm ressies at 'OHana. Since we are always early, we arrived around 4:00 so we just walked around.
We noticed around 4:30 or so people started lining up at the empty podium. So I quickly got in line because it was already starting to get mobbed.

I was probably 6th in line & there was already probably 50 people behind me in line!

By 5:00 the line passed Kona Cafe and was almost out the door to the Monorail!! (seriously not joking)
At 5:00 the CM made an announcement: line up on the LEFT for reservations, on the RIGHT for walk-up. It was a mob scene. Alot of people didn't have reservations and they were being told ridiculous wait times-2-3 hours (and that was for the first few people in line!!) Others were being turned away. People were not happy.
I had reservations and I didn't get to the podium until 5:20!

I can't even imaging how long it took the others to check-in.
My reservation was for 5:00 but the CM was alternating between the lines and it was utter chaos.
We finally got seated around 5:30. Personally, I would not risk it by just showing up w/out a reservation. It is way too busy of a place.