We just did this on Wednesday. We were not impressed with the actual dessert party part of it. The tables on the upper level are packed in. It felt like an overally crowded cafeteria, complete with bad lighting. A CM finds a table for you, thank goodness, because you have to squeeze between tables. The deserts were decent, but hard to enjoy in the atmosphere. By the time we got there the crackers were gone as well as most of the cheese.
That being said, the actual wishes portion was fantastic. The area isn't crowded at all. everyone rushed to the rail, but we headed to the back rail so we could lean. The people that headed for the front rail had non-party people right in front of them on the other side of the rail. We were completely unobstructed because there wasn't anyone in between us and the people all the way up at the front rail.
You can stay and watch the castle show, and we did. At that point it is no longer roped off, but very few people came in.
Honestly, on a really busy day like we had, I would consider doing it again just for the space, but I truly think I would skip the dessert portion and consider just getting my wristband and going in to the viewing area when it was time. We found the dessert portion that miserable. We've done the Star Wars party and loved that, this is nothing like that.