I just recently ground up a whole packer without trimming the fat. As far as using as a replacement for ground beef, it is not very good at all. I have had far better luck with trimmed brisket.
I can only assume it is a difference in personal tastes? For browning in a pan, the resutling product was too fatty and chewy for my wife or kids to eat. They basically skipped dinner that night (tacos) because the meat was too gross for them.
Thinking the meat may be better if cooked in a liquid environment, I did a test with a big batch of chili. I put the raw ground packer brisket in with some beef broth and heated until the meat came apart. Then made into chili. The result was an extremely greasy chili and the meat broke down into such small particles the meat had no discernible chunks at all and resembled more of a sludge. i was able to salvage the chili by straining out the solids from the liquids, chilling the liquids and removing the hardened fat from the liquid, then recombining everything. I added pinto beans for texture as teh meat was too fine. It was more of a chili sauce really.
Again, in my humble opinion, if you wish to grind up brisket, trim it up. That is what I did previously for hamburgers and it worked great. Grinding up a whole packer makes for a fatty chewy product that isn't suited as a ground beef replacement (for me anyway).
I have heard some suggest grinding it twice, and have heard others say to trim the fat off. I an definitely leaning towards trimming the fat off for starters. For the hamburgers I have made in the past with trimmed brisket, I never had to grind twice for good meat, so I don't think that is the important thing.
I am only posting this so nobody else makes the same mistake I did.
If you make it into burgers and cook them very well, say 195 internal, then this stuff will probably be fine. I made some meatballs with the ground packer and cooked the chit out of them like burnt ends. They were really good. so it could just be my fault for trying to use it like ground beef. But I still think this warning should go out.