We live in So Cal so Long Beach is easy to get to, and thus have cruised on the Imagination and Inspiration (both Fantasy class) more than once. We were excited for Long Beach to finally get a new ship, and sailed on the Christmas 2019 sailing (Panorama's second "normal" sailing).
Embarkation was amazingly smooth. We drove to the port and arrived right in the middle of our scheduled arrival window (12:30-1:00pm) and it was a breeze getting checked in, through security, and onto the ship. The new terminal that Carnival built at Long Beach a year or so ago has helped a lot in improving the embarkation flow (not that I found it much of an issue in the past)
The downside to cruising over the holidays is that everyone else has the same idea, and the ship was pretty full. By the time we reached the Lido deck to await opening of the cabins and grab lunch, it was so full that we had to sit outside on the pool deck. No big deal.