If you are contemplating a startup or you're already doing a startup I would say it's worth attending. Outside of the interesting talks (which will most likely find their way online anyways), it's a great opportunity to talk to other like-minded people and hear about what sort of stuff they're working on.
What's the general atmosphere of the event? It's intense (lots of content), and no-nonsense. Every speaker had something valuable to say, and they didn't talk down to the audience at all.
I found the day motivating, especially DHH's talk and just chatting with my peers. With the exception of one person, everyone I met was a hacker and was working on something.
Is it similar to other conferences you've attended? Not really, one big difference: the overall quality of the attendees. I'm a shy person, but there were so many people talking about writing code (vs. big idea people) that I felt like I fit right in.
Is it simply a day-long sales pitch for YC? The only real pitch-man the year I attended was Jeff Bezos, his talk was a pitch for AWS. However, he was so impressive during the Q&A that I can forgive him.
It is well worth a few hundred bucks to attend... and ridiculous in that it is free.
So yeah, apply. It's worth it.
Startup School is my favorite tech conference-alike-thingy (not really sure I'd call it a conference), by far. I'd be going this year if it wasn't in the middle of my trip to Nepal.
No idea how many applied, of course. Given the HN audience, it could have been thousands.