First off, I want to just say thatI think YC is a very good program for younger developers and/or those who know the problem space they want to develop in but haven't quite worked out the firm business model.
However, my negative experience with the program is that I think the funding for the equity you give away is a very poor deal for founders. $20k/$6k a founder doesn't last long and it seems to me that you could raise similar/more money on much better terms elsewhere.
Sure, you get the incubator program as the 'meat' of the investment but it's like saying working at Google is best because you get the wonderful free lunches... I can get better job + pay elsewhere and sort out the lunches on my own (read: where 'lunch' is the network, connections, feedback on my startup, that YC offers... others can provide that too etc).
I'm expecting this will get down voted because most people here are very pro YC. I am too, I even went to Startup School last weekend. I just wouldn't want to take funding at YC over other angel and seed routes at a better valuation (or even better, delay valuation entirely and take a note instead).