Pros: Friendly, beautiful birds
Cons: Leg problems
I got two Dark Cornish from my McMurray order this spring. One of them turned out being a runt with bad legs. She was so sickly and weak I did not think she'd make it. She did however live and turned out to be an extremely friendly, sweet, goofy, funny pullet. She still had bad legs though and an overly HUGE chest that made her tip forward. She was unable to fly or roost properly and we had to make sure a special roosting box to sleep in at night. She was very unwieldy and clumsy and acted mentally "slow" or "off." At 4 months old I found her dead in the morning with a broken neck. It appeared she flew headlong into a wall.
The other Dark Cornish was not very friendly until she came into maturity at 5 months (20 weeks) and started laying. The first 5 months of her life, I could barely get near her. Now she's my best little buddy! She sits in my lap, and even flies up onto my shoulder when I walk into their run. She is extremely busy and is constantly digging and rooting for morsels, although she is confined to the large run and chicken house. I can't free range because of hawks.
I had read that Dark Cornish are not good for a mixed flock because they are so slow moving, but that's not been the case for my hatchery bird. Perhaps if one gets more extreme show type genetics from a private breeder that would be the case. Personally, I prefer my hatchery quality DC because she DOESN'T have that extremely squatty appearance and legs so wide she can hardly walk. I don't believe I would want a chicken that can't breed naturally or get around because they are so short and fat.
My DC hen so far is the first and only of my pullets to be laying. She has laid an egg a day for 3 days in a row, and none of my other pullets have done anything yet. I have read that Dark Cornish are very poor to non-layers, but again I think hatcheries select for egg laying traits and not show traits. If you want that extreme stance of the super short, squatty bird that waddles around, I don't think you'll get that from McMurray. My hen is very fast, agile, and "gamey" but is extremely friendly, docile, and sweet!
My only regret is that I didn't get a few more of this breed with my order!