We were there two weeks ago and made lunch reservations for Mythos at IOA and Finnegan's at Universal. Wouldn't have needed the Mythos reservation as there were some empty seats, but Finnegan's had people waiting as we were leaving. They were pretty busy. We enjoyed both restaurants.
At City Walk, we never made a reservation but did encounter brief waits at Cowfish and Antojitos. If you're staying onsite, you can take advantage of priority seating at these. Surprisingly, no wait at Toothsome or NBC Sports.
For all these, we dined at prime meal times - 12pm for lunch and around 6-6:30pm for dinner at City Walk, also near park closing time for Universal.