I think POR-15 is as great as long as the instructions are followed EXACTLY. Cheat even the slightest bit and it will fail. I have done two Z tanks and both are doing fine so far. One was done a year ago and the other is going on four years. To do the job right, it takes about a week of back breaking labor. On both of my tanks, I had to add many pre-cleaning steps prior to even starting the POR process. I rinsed the tanks with water and lengths of chain at least 50 times first to get the majority of the rust out. After the water only rinse, I used water and muriatic acid to further prep the tanks for the actual POR treatment. I'm pretty sure I did a thread on it last fall. I'll look around and post the link to the thread when I find it.
EDIT: I must not have posted pics. Here is my 260 tank before and after POR-15
This is the inside of the tank after the Z sat in a barn for 15 years or so
After a few water rinses, this is what I got out
After a week of cleaning, here is the same tank inside ready for the sealer step
Finally, after sealing