There is some difference in flavor among breeds. The Cornish X's are a little bland, a dual-purp breed of the same age will be a lot smaller, but have more flavor. I've butchered both at 12 weeks, that being the minimum to have any size at all on the dual, but still only about 2-2 1/2 lbs., dressed. The Cornish X was huge, of course, maybe 6 lbs. My little scale tops out at 5 lbs. it was well over that. 6-8 weeks old is optimal for tenderness, but up to 10 weeks is still tender enough, if you cook it slower than usual. After that, you get into the realm of the crock-pot bird, which is still yummy, you just have to give it time to get tender. The older the bird, the longer it takes. Brining an older bird will somewhat mitigate the "stringy" factor, and the meat won't be so dry. Older birds are good pressure canned, too, which is what I'm doing with the next batch of extra roos we have.
My guess about the people who said any breed of bird "tasted horrible", is that it was older, tough, and they tried to cook it like a fryer. Or maybe they are so used to that bland, mushy, 6 week old Cornish X meat, that they don't like chicken that tastes like chicken.