It's too soon to say for most startups in the recent classes. From earlier rounds, there have been successful exits like AppJet, ReMail, Reddit, Omnisio, Zenter, as well as startups that appear to be thriving like Loopt, Justin.tv, Weebly, Scribd, AirBnb, Xobni...too many to count. YC started in 2005, and it can take 5 years sometimes to see if a startup is successful, so we'll have to wait and see who makes it big over the next few years.
Judging by the success we've seen so far, it looks like YC is getting better and better each year. We'll see a few solid exits per class in the years to come, and I'll bet it won't be long before we see a massive acquisition on the scale of YouTube which will really catapult YC into the spotlight (what's 6% of $1.65 billion?)