Got my matte black 7+ today, wen to Apple Store and checked out the jet black. I will say, it does have a sort of plasticy look to it, like the iPhone 3G back. And it was super greasy, this was at about 11:30am so the store was only open a few hours and it looked terrible with all the marks and greasy finger smudges. And also was surprised this morning when I got my matte black that it was actually black. Looked kinda dark grayish in pics and videos.
But I will say, the jet black looks almost like an entirely new design in comparison to the others. I wonder if the glossy look was their original plan then they scrapped it due to the grease/scratches. I am sure the JB looks amazing when untouched too. Probably is more exciting for 6S+ owners, in terms of that new phone excitement than the more traditional matte finishes.
Definitely don't regret my choice, but wanted to give my unbiased opinion. I hate fanboys/fangirls and feel I'm fairly subjective on matters such as this.