I've had one at John's before and it was great. I think the broccoli rabe almost makes the sandwich...along with the juices. Are they served that way all over Philly?
I think that if you go back just a handful of years, it was a specialty of a few places in South Philly. But the sandwich has gained a lot of notoriety in recent years and has become more common around Philly. But a lot of places that offer it outside of the few legendary places like Tony Luke's, John's, and DeNic's aren't in the same league as those guys. I ordered it at one place where they just took some pre-sliced roast pork and dipped it in juice. It was a very mediocre sandwich.
One place that I never see mentioned anywhere, but which serves great sandwiches in general, is Pastificio, which is in pretty deep South Philly. I've never actually tried their roast pork. I've ordered hoagies and meatball sandwiches there. But I would bet they make a good roast pork Italian. www.pastificiophilly.com