I've never heard of a negative YC experience either. I'd guess I've met at least one founder from roughly 40% of all YC startups. This includes some companies that have failed or been absorbed into other YC startups.
I can't track it down right now, but there were allusions made on HN awhile ago about at least one person who wasn't thrilled with the program. I'm pretty sure there aren't no bad experiences.
That said, I've gotten to talk to a lot of people who have done YC now, and everyone I've talked to --- against pointed questions from me! --- has been thrilled with it.
YC was/is certainly an amazing experience for us.