Here's a slightly longer write-up.
The concert was about two and a half hours, including the encore of One-Winged Angel. As I said, Nobuo Uematsu joined the choir, and the audience was also invited to join in (Arnie Roth, the conductor, said "You only need to know one word", to which everyone replied "Sephiroth!") but I couldn't hear those around me making much noise.
Just prior to the encore, Uematsu and Roth performed an (apparently) improvised version of the victory fanfare with a kazoo each. The instruments had been given to them by a particularly excited fan, who must have paid for VIP access at the concert, including the chance to meet the pair of them. She ran up to the edge of the stage to join them for the brief performance, which was well-received by the audience.
I can't remember the whole set list, but the one
here seems accurate. The three pieces with a lead vocal part - Melodies of Life, Kiss Me Goodbye and Answers (which I hadn't actually listened to before) - were sung by Susan Calloway, who was great. Vamo' Alla Flamenco and Dear Friends were performed as guitar concertos, and in the latter, the guitarist played a note too early and had to be quickly stopped by Roth. The Chocobo medley got a few laughs out of the audience and was a good way to go to the intermission.
The Opera song from VI was my highlight. Roth said that they were playing the "full, extended version", which included a narrator, though I'm not sure exactly how much longer it was over the "standard" version. There were three classical singers, one each for Maria, Draco and Ralse. I'm not normally a fan of opera - probably because I have no idea what they're singing (
"it's just vowels!"), but this really got the hairs on the back of my neck to stand up. It was a wonderful performance and drew the first of three standing ovations of the evening. I really have to tip my hat to Uematsu for having the vision to write such an ambitious piece all the way back in 1994 (or probably even earlier), despite the limitations of the hardware he was working with.
TL;DR: 9/10, would go again. Would have been 10/10 if the tickets had been about 20€ less or if the concert had been two or three songs longer. There are still many, many Distant Worlds songs that they didn't play, which I'd love to hear.