I've used it and it's great. I mainly bought it to use on part of the rear floor pan and under the battery tray in my XJ, and parts of the unibody underneath my Mustang. I had some areas that were pretty significantly rusted and wanted a product that would not only prevent rust from coming back, but also help reinforce areas where the metal was thin or pitted. It did exactly what I was hoping for, it cures super hard and you can apply it much thicker than paint, especially if you do multiple coats (I think I did two). I'd definitely buy it again.
These guys are right, following the instructions precisely and doing the right prep work is important if you want the best results. I used POR-15's Marine Clean and Metal Ready products as recomended before applying the POR-15 itself. I wasn't sure if they were really necessary so I actually called the company up, they were very helpful and explained that while you don't have to use them, you will get much better results if you do. The Marine Clean gets the metal perfectly clean of any grease or residue and doesn't leave anything behind, more so than other degreasers apparently (it is one hell of a degreaser); and the Metal Ready etches the surface to give the POR-15 something to stick to. You can skip the Metal Ready when applying POR-15 to already rusted surfaces (although it doesn't hurt to use it) but they say it is absolutely crucial to use it on virgin metal. In my case I had removed the loose rust using a wire wheel on a drill, which also took paint off of the surrounding areas, so I used it. It's a bit of an investment to buy all three products, I think I spent about $65, but they go a surprisingly long way. I have plenty left for future projects so it was totally worth it IMO.
Buy good rubber gloves, have some rags handy, and wear clothes you don't care about (shoes too), because this stuff does NOT come off. If it gets on your skin you'll probably be wearing it for a couple weeks. I did my floor pan almost two years ago and I still have a quarter sized black spot in my driveway where I dripped some on the ground. I had dripped some onto my muffler from an open drain plug hole and it was still on there when I replaced it a year later.
Also worth knowing is that if you get POR-15 around the edge of the can it comes in and close it up like that, it will seal the lid to the can and you'll never get it open again. If you don't close it up completely air tight it will cure and you won't have any more usable product. Since I bought the smallest can they sell, I use a tablespoon to dispense it into a separate container to brush it on from, and then close up the can immediately. I haven't opened it in couple months but it seems to have a pretty long shelf life as long as it stays closed up air tight.