If the stuff is used right and not abused it'll last for a very long time. I've never heard of any failures when the product is used right. Apply it over a good epoxy primer for excellent adhesion and durability. Although it specifies that it can be used over an exhisting finish just remember it'll only be as good as what's under it. If you cover old shrinking laquer and deteriorating paints then expect it won't hold up to it's true potential. The only person I ever heard complain about any of Evercoat's polyester primers said he had some spots that stayed soft and didn't harden--and I assume he must not have mixed it completely and didn't strain it so chalk that one up to user error. For years I shunned polyester primers which was a big mistake, it is a very good product when used properly. Featherfill G2 gets my vote.
G2 is available in black, grey, and nuetral. The black is especially usefull on underbody components. Start with a sandblasted metal surface then coat with black epoxy, then black 2G to fill any pits etc, sand and apply another coat of black epoxy then your chassis black or satin black urethane singlestage. The beauty of it is all materials are black so any rock chips and scratches will show very little down the road.
Another area where polyester primers really shine are small parts that are pitted. Parts like convertible top bows, windshield openings, and any parts with intricate shapes. These parts are a PITA to try and glaze even with a brush but spraying on polyester primer is easy. JMO