I know this is an old post, but wanted to chime in, just on the chance someone out there is interested in NJs. .
They are sort of flighty birds, but can be very sweet at the same time. They do like to fly, but mine only do it when startled and as soon as the 'danger' is gone, they're right back down with the rest of the flock.
My roos are all 'manly men' and started crowing very early..I can't remember the exact age, but they weren't even moved out to the outside brooder, so no more than 6 weeks.
They are good layers and foragers, I've not set down and figured my birds rate but have been told they have one of the best feed to egg conversions. A lot of other areas cross them with leghorns for commercial egg businesses.
They are also auto sexing. Cockerels will have a light colored patch on his head and it's irregular and will often go towards his neck. A pullet has a round spot on the top of her head.
They are extremely hardy breeds, even if extreme line breeding is done. I don't know about being "resistant" to disease or illness but they are more hardy than any other breed I've dealt with.
Some of the first birds I had came as shipped eggs clear across the country and we're even hardy as embryos