Before she was picked up for the Toronto production of Les Miz, Samantha Hill went on as the Christine alternate on Broadway in POTO. I saw her in, I think, her second performance. I realized immediately that she was going places. I also think Mary Michael Patterson, as Sierra Boggess's alternate, did a better job than Sierra. (I saw them both in one day, with Norm Lewis as the Phantom).
Before Norm Lewis became the Phantom, Hugh Panero seemed to have gotten tired of the role, and didn't have a lot of energy anymore (except for the 25th anniversary, which he nailed). I saw Greg Mills as his understudy, and was blown away. I also saw the new tour on a day when Cooper Grodin was out. I don't remember the cover's name, but he was fantastic also.
One minor problem that I've noticed with Phantom covers: They seem a bit tentative with their footing onstage, whether it's the Bway scenery or the tour. Greg Mills balanced himself on the gravestone and the tour sub walked very carefully down the twisting staircase. I couldn't blame either of them -- the sets in POTO are downright dangerous. I met one of the Phantoms at another show and he told me his mother almost had a heart attack when she saw him descend in the angel.
Audrey, the Phantom Phanatic, who nonetheless would rather be Jean Valjean, who knew how to make lemonade out of lemons.