Accelerators vary a great deal in their results. In Seattle, we have several. Techstars, 9 Mile Labs, Fledge, Founders Institute, Kick.
Some follow the Tech Stars model closely, others are further away.
When Accelerators work well, they have a few things in common:
1) They have deadlines to accomplish tasks, so you have a built in form of accountability with consequences.
2) They have mentors... But more importantly, they have mentors who are entrepreneurs who have succeeded, who have the capacity to invest in the company.
3) While YC is not a "resident" accelerator, Techstars is. As a resident accelerator, they have everyone co-located, so you get to watch the cohort teams in action. Nothing like peer learning from people on the same road and at the same stage as you.
4) Cohort members are typically wicked smart
5) The demo day at good accelerators attracts very interested investors,who are actively looking at your work.
6) There is a significant amount of expectation and feedback in good accelerators, and they hold you to the task of making progress.
7) They typically truely get the Lean Startup pathway, and help you understand it in practice, not just in theory.
8) It is not about the money that you get going in, but the funding you get leaving. If you don't get funded, then you have missed one of the great advantages of the accelerator model.
9) There is a wide cohort of other startups at your stage who you will gain deep relationships with, who will be able to help you along the way after you have left the accelerator.
10) If you have engaged the mentors well, they will be on your side long after you have left the program, and they will open doors, give you feedback and otherwise help you reach success.
If you don't have a good team or evidence of traction in the market , you are not likely to make much progress. The goal of the accelerator is to give evidence that the company has a team that can perform (on the business , not on the product) and evidence that the market cares that you perform. It brings a large number of eyes to your work, and if done right, creates significant amounts of social media and press for you.