It would be counter-productive to say anything bad about seed investors who own 10% of your company, on their news website.
We had a great experience with no regrets. Apply now if you're on the fence. Having said that, my criticism is that, like any startup, YC has some growing pains while it scales.
In particular, our experience (S2010, 36 companies) wasn't as intimate as I had hoped it would be. But they're working to address that, which was a learning experience in itself. You get to see how smart people try to scale an operation while preserving what makes it special.
Relatedly, I also didn't expect YC to feel internally competitive at times. Resources can become limited, from the basics like seating and food, to the important stuff like the time and attention of speakers, investors, and PG himself. With that many companies, there's also inevitably some overlap among product ideas.
However, overall it was much more cooperative than competitive, and I welcomed a little friendly competition. The resulting "coopetition" is a balance I think they'll need to watch as they scale.