We had three things we where hoping to get out of joining an incubator and i/o seemed to offer these things (plus it was in the mission in SF, which rules):
1) Ability to hang out with other people doing start-ups. My co-founder and I had been coding alone for the last few years and we really wanted some people to develop relationships with who were doing the same types of things as us.
2) Introduction to the top angel investors in the valley (to get our next round of funding). The guys at i/o definitely gave us a huge amount of introductions. There were 150 angel investor / VCs at our demo day for only 6 companies (that is a pretty badass ratio!)
3) Help with our product. While we thought we knew what we were doing, the guys at i/o taught us a lot about how to build products people want.
Overall the experience for us was great. From the time we joined in June we went from around 200K users to over 1.7 million users today. We just recently wrapped up our seed round of funding (400K) from some of the top investors in the valley, including Charles River Ventures and Dave McClure's 500 Startups. Additionally we made friends that will last a lifetime.
i/o has one of the best track records I have heard of as well. There were 6 companies that joined, 3 of those companies (including us) went on to raise a solid seed round, 2 were acquired and 1... well is pivoting, so we will see what happens with them.
Jim Young (Hot or Not) was the main i/o partner who got us to join. I will never forget the way he described incubators (as it was one of the main reasons we joined as opposed to waiting for the next YC cycle). He said, "Incubators are like martial art houses, different incubators specialize in different things depending on what you want to do." I found this really true with i/o, because they seemed to specialize in costumer facing startups based around social media and entertainment. That was exactly us so it was a perfect fit.
One main point is this, there are many good people out there who can get you to where you want to go. YC is just one of many routes to what I think is every entrepreneurs goal: Building a successful company that greatly benefits your life.
Enjoy yourselves!