Yeah, I actually tried Por-15 for the first time this year on my E30 back in April, but I wasn't exactly impressed with it. Usually stuffs rusts from the inside out, so the seams, channels and whatnot you cannot get into. Even with a few coats of Por-15, and half a year later you can see traces of the rust coming back out again. It only works IF you can coat the entire area that is affected 100%, otherwise it is no better than regular paint. If you could dip the car, or at least the bottom of it, then yes I would agree that would definitely be a better option. But insides of rockers, frame rails, etc I can't get to.
The surface rust areas like the outer body I will just wire wheel, and primer/paint over. With a few good coats of paint (both sides), it should seal it well enough to stop the rust. Tremclad FTW!