Judging by the two Black Jersey hens I have, which are around three years old now these chickens would seem to be a good dual purpose heritage breed to have on the homestead. While I keep them for eggs, they are not prolific layers, typically giving around 5 or 6 eggs (each) a week during the spring and summer. However laying will taper off during the winter significantly.
While bred to be a large table bird, mine are not at the top end as far as expected weight for this breed is concerned. I believe they are somewhere in the range of 6 to 7 lbs.
Their maturity comes a bit later than your production types which is to be expected for a heritage breed. They may not begin laying until around a year give or take.
When it comes to their place in the pecking order, they're on top of it. Between the two they share leadership over my small flock of Buckeyes, one Black Copper Maran, and one up and comming Black Sumatra cockrell. However one of them is dominant over the other.
These Jersey girls demand respect, and require submission so as long as the other birds don't challenge that or submit all is well after (they remind any protesters who's boss.)
They do often wander out on their own, but will sometimes pair with their flock mates to hunt down bugs, grubs, worms, and critters. They're good at finding what they need out to pasture including grass and keen to alert for a snake or a hawk over head.
Purchased from Meyer Hatchery, these two were and remain very healthy. One survived a vicious dog attack which left her with punture wounds through her back. With the help of my Vet she has fully recovered as if it never happened.
They're not big fliers, but will roost high if given the opportunity. I generally don't worry too much about them as they take care of themselves, and know their limitations as far as weather and temperature changes or extremes. When it's hot they'll dig down in the shade. During the winter they fluff up and conserve engery, but do require plenty of food and water if weather prevents them from seeking it out themselves. It's always a good idea to have food on hand regardless of the season to make sure they're getting what they need to thrive.
When it comes to people they're not shy, and like to be fed. If you do that regular enough they will come to expect it. They can be very vocal when they want attention or you miss feeding time.
I recommend this breed for people in the Midwest with cold temperatures, who free range. Definitely a breed worth preserving.