I have both cornish and a ranger type meat bird. I actually am finding that I prefer the cornish. I think they get a bad rap. Mine aren't any less intelligent than any of my other chickens. Yeah, they move a bit slower because they are getting big, but they eat grass fairly well, still try to run around and fly a little.
More than anything, it really just makes me sad that, as a society, we have done something like this to an animal; genetically conformed it to the point that it can't even live a life of any length at all.
I'm not sure how old mine are because I'm thinking they might've been a little older than I thought when I bought them, but I'm just going to try and go by weight to decide when to butcher. I'm feeding during the day only and putting them in a dog kennel (a big, wire one - I only have 6 cornish cross) during the night.