Anecdotally, most. I've backed about 50 projects and none have failed to deliver, though plenty have been delayed.

I funded way more than 50. All delivered, most of them at least 3 months delayed.

I'm pretty certain now that I can predict the delay outcome just watching the video. I started writing down my predictions over 1 month ago. We will see how it goes...

I think it's a bit early to say since Kickstarter only really took off about a year ago and many projects have longer schedules. That said, if the biggest failure Kickstarter has had so far is a 100k board game, then I'd call it a roaring success. I'm fairly certain that we're talking >99% delivery rate, even if they might not all deliver exactly what the backers expected.