When they first put tickets for the Producers on sale (around April, I think), I bought one... the calendar was wide open, so, I decided to get it for my birthday weekend (September) because "why not?"
Around about in July or August, I think, they called me and let me know that Matthew Broderick would be on vacation for the week encompassing my date. I could get a refund if I wanted... but, I couldn't exchange the ticket because the rest of the run was sold out.
Given the choice of not seeing the show at all or seeing it with Broderick's understudy, I chose the understudy. But I was frustrated.. especially because I'd had no limitations on choosing a date and if I'd known when I purchased the ticket about the vacation I would have simply chosen another day.
Actually, the same type of thing happened to me with History Boys. Right after the Tonys, when they announced they were extending the show, I bought a ticket... particularly wanting to see Frances de la Tour. The next day they announced her vacation, starting the day before the day I was going.
Both shows were still good and the understudies great... it was just frustrating.
I'm infinitely less bitter when I see an understudy due to illness or something else unexpected.