I'll agree with the general consensus here, it just truly depends on whether your personal conviction to see the show is star-dependent or how the health of the lead is affected (or, true, how capable the understudy is.)
I saw NICE WORK IF YOU CAN GET IT last year the week that Michael McGrath was out, and his understudy was just as good as I could imagine McGrath was. It was clear to me why it was a Tony-worthy role, (which is to the credit of the understudy for being as impressive) and having seen McGrath before in SPAMALOT years ago I can easily imagine why he won.
Earlier this summer, I saw AROUND THE WORLD IN 80 DAYS off-Broadway with an understudy who was very obviously underprepared and it eventually became clear that this was his very first time performing the show for an audience. Other actors were visibly improving to cover for him and many of his lost lines (and excellently, I might add; I don't believe that original cast is still together as Bryce Ryness has gone on to FIRST DATE at least but that was a really charming ensemble) and at the end of the show the other four actors took a moment to applaud him after their bows. That was a case where clearly there must have been some sort of emergency, because he should never have had to go on under normal circumstances without (seemingly) any rehearsal at all.
It's hard to generalize because it is so variable on a case-by-case basis.
Words don't deserve that kind of malarkey. They're innocent, neutral, precise, standing for this, describing that, meaning the other, so if you look after them you can build bridges across incomprehension and chaos. But when they get their corners knocked off, they're no good anymore…I don't think writers are sacred, but words are. They deserve respect. If you get the right ones in the right order, you can nudge the world a little.