Probably not an understudy situation, since I believe she actually took the role after Kelli O'Hara left it ... but by the time I got to the Lincoln Center revival of SOUTH PACIFIC, Laura Osnes was the Nellie Forbush, and I loved her more than any Nellie I'd ever seen. This was near the end of the run, not too long before the PBS telecast for which O'Hara returned, so the show was fresh in my mind -- and as I watched the telecast, I thought "how ordinary". I'd so loved how young and fresh Osnes had seemed to me, and I didn't think O'Hara brought that particular magic or anything else unique to it. I'm sure I'm uttering blasphemy here, but such were my impressions.
In all fairness, the whole telecast was a lesson in how much theater magic is lost in translation to the TV screen. For one thing, this one was notably rushed for the allotted time, and some wonderful moments on stage were literally eviscerated through being truncated. Of course, had I not seen the production live, I would most likely have been thrilled with all the good that was there on the screen. And especially in the speakers.