I have been raising DP Heritage birds and have also done Cornish Cross and Freedom Rangers. My opinion is that out of all of them I like the Heritage DP breeds the best. I raise Silver Gray Dorking, Australorp, and Brahmas for my meat birds so the ones I get each year may be purebreds or they may be any combination of the above breeds. I keep my Brahmas in one coop with two Brahma roosters, and keep the Dorkings and Australorps in another with two Dorking roosters. They are primarily what we eat here.
The Cornish Cross we raised were better than store bought definitely but I agree with the OP and others who have stated the meat was bland and soft. I also didn't like raising them as they ate and ate and when they ran out of food they would break out of their pen and go after the layers foods or the eggs or anything else that wasn't nailed down. I guess this means they are good foragers but they don't forage like my layers do.
The Freedom Rangers grow a little slower and taste about the same as the Cornish Cross do but I had the same problems with them as I do with the Cornish Cross. They would eat all the food and then go after anything that wasn't locked up. So eggs, chicks, human legs were all easy targets. Others have said that they are friendly and easy going. That wasn't my experience. They were free range from the time they were large enough that my other chickens wouldn't kill them so it wasn't that I was keeping them penned up or not giving them enough. I was going through a 50lb bag of food every 2 days if not more.
The Heritage birds are much calmer in my opinion. I can run the ones I am going to be eating right with my layers and they are mellow and easy going. They have access to fruit trees as well as bugs and pastures for grains and grasses and they go out in the morning and always come back with their crops full (and I still get eggs). As far as cooking the heritage birds. I have roasted, grilled, slow cooked, fried etc all my birds (up to 6 months for roasting frying or grilling, up to 2 years for crock pot so far) with no problems. On my grill I do indirect heat so I turn on a burner to the side of the bird and the bird sits out of the direct heat but still gets browned up nicely. Fried chicken I usually brine or soak in milk or yogurt before adding the coating and frying. So far I have not had a tough meal from this. Roasting is lower temp than store bought birds like 275-300 in a closed dutch oven usually with veggies in the bottom and a little fluid of some kind. Skin gets crispy and it tastes wonderful.
It is definitely different cooking Heritage birds than cooking store bought or cornish cross birds. I agree with Natali that you definitely need to look into how to do it before you give it a try. I am not saying that I doubt what others are saying I can only tell you what I have experienced so far with the birds I have had. Maybe it's the breeds I am using or how I prep them ahead of time, I am not sure but so far I have had no complaints from the family that their meat has been tough, bad tasting or inedible in any way.