I was in MassChallenge '10 and it was a great experience. One big difference between MC and most incubators is that it's a non-profit. You get free office space, access to a great set of mentors, and, if you're one of the top teams, you get a share of $1m. Everything, including the money, is non-dilutive and comes with no strings attached.
I am noticing that many state governments are also sponsoring various forms of incubators, which also offer non-dilutive funds. I think that can be just what some start-ups need.
I was in LaunchBox DC (which has since moved to NC Raleigh/Durham area) it was a very positive experience. The advisors brought in great connections: Steve Case, Tim O'Reilly, various startup CEO's, etc for weekly round table discussions and advice.
The overall outcome was a third of the class ended up either getting acqui-hired or taking additional investment, a third is still bootstrapping, and everyone else scaled things down to a side business or just faded away.
Kincast is in Sproutbox http://sproutbox.com/ right now. The experience has been great. You instantly get a great UX person, designer, and a few developers without having to recruit and hire people. You don't get lots of cash, but the instant access to more engineering resources is very nice.
I can't say from personal experience having only met the team behind it and many founders but can refer you to a number of people that know a lot about it.
I've heard it's on par and similar to the 500 startups accelerator in Mountain View and they work together quite a lot from what I've seen.
