I've gotten Por15 to work well, but I would not use it again. I know that sounds strange.
I didn't set out to do it, but my resto project has become a product comparison between rust inhibiting paint. Its sitting in an open car port, body on stands above the frame. The frame is covered in Por15 and its top coat. 7 years later, it is still black, so it can work just as advertised. However, the frame was a rough, rusty-orange surface which seems ideal for Por. On smooth metal, ever after their etch, I have seen it run off like water on glass.
That being said, my real negatives about it are:
1. It is very "plasticy", even after setting for a long time. If you really scrape it (as I did moving the frame), it peels off like a big sticker unless the metal is really super rough underneath.
2. Cost. One of my floor pans is done in Por and the other in "Masterseries". I have inadvertently setup a side-by-side comparison and they have been aging for like 5 years now as my resto grinds along endlessly.
The Masterseries is holding up just as good, at a lower cost. It also sticks to the metal easier when painted on. I don't believe you have to etch with master, but its been years so don't quote me. The car's firewall (done in masterseries) faces directly into the rising sun and gets blasted till about noon. It seems completely unaffected (the Por15 topcoat will dull and get hazy after years of sunlight). I havent had a major scratch in the master, but it doesnt appear to be so "plasticy"- still unsure if it would peel like the Por does.
So, in my experience, Por is a good product if properly applied, but MasterSeries is better at a lower price.