I never got into Y Combinator (I think I applied 4 times but I'm not sure) but I'm now the CEO of a rapidly growing, well funded startup that actually SERVES an increasing number of YC companies. The way I always thought about YC was that it is ALWAYS worth applying because the exercise is good and the upside is high (and the downside minimal) but getting into YC should never be "part of your plan" - there are too many other great applicants, plain and simple.

It's the same as how no student should ever PLAN to get into Harvard, and Harvard even has SATs and GPAs to rely on (fairly objective stats around which you can make a guess about your odds) while YC has only heuristics based on experience and founder reputation. Never judge yourself based on something that has an open application process - it doesn't define you, it defines them. No reason not to apply, but take it as a bonus if you get in, don't take it personally if you get rejected.