We had UDP on the Symph last March
I went to one of the Kiosks in the promenade right after boarding and made all the reservations.
The entire process seemed very inefficient and slow, it took around 30min.
It was kind of a hassle for the guy going from screen to screen checking availability.
Like other said, most of the good times were taken.
But after I told them who I was, we ended up getting everything done.