For a minimal time investment x maximum meat production, they really can't be beat. If you are keeping them on with plentiful space+available forage, they have no trouble walking, dust bathing, pecking on feet and generally being chickens for the 2 months they are alive. Last year ours went to 9 weeks and were 10+ lbs at slaughter. You just can't have that sort of turn around time with heritage breeds.

Photo- our 5 week olds, poking around while I dismantle the compost.