I've used POR-15 on my old Dodge trucks. Works very well, but prep is a pain and so is painting over it...so I've taken to using it only on areas that don't require topcoating or that are not exposed to sunlight. On the plus side, it holds up very well and so far not a bit of rust has reappeared on treated areas.
Auto Restorer magazine recently did a test of about a dozen rust conversion preps. The one they rated absolutely tops by a large margin was Rust Encapsulator by Eastwood. I've never tried it, but with a recommendation like that I sure will in the near future!
Pete