I always try to approach seeing understudies as the chance to see performers whose work I don't happen to know but who I hope will grab the chance to show how great they can be.
Right now on Bway, Patti Cohenour covers wonderfully for Victoria Clark in LIGHT IN THE PIAZZA, and as I've said in another thread, David Burnham is a great Fabrizio (he goes on for a week later this month, I believe, so go see him!).
I'll second the poster above who said that Sarah Jane Everman was a super Glinda in the Chicago WICKED. She nailed the role as far as I was concerned when I saw it back in October.
Once upon a time, I saw Jana Schneider cover for Tyne Daly on the pre-Bway tour of GYPSY, and she gave a very strong performance -- and talk about having to overcome audience disappointment!
And once I *heard* but didn't see an understudy in a performance of CABARET: Joel Grey had laryngitis and couldn't sing or speak, so he mimed the Emcee while his understudy supplied the vocals from offstage somewhere!
"Sweet summer evenings, hot wine and bread /
Sharing your supper, sharing your bed /
Simple joys have a simple voice:
It says why not go ahead?"