I run Flowmaster's 00-01 V8 catback kit on my 5.0. It is a decent system, but I wish that I would have known that it doesn't fit the Mountaineer, but does fit the Explorer. I was able to get the hangers bent so that it would fit the Mountaineer, but I still have a problem with the pipe hitting the right rear leaf spring.
Anyway, it has a nice deep, throaty sound, but almost too quiet for me. It's a 50 series "Big Block" muffler. I'm thinking about ditching the entire system altogether and getting a custom system with dual Flowmaster 40s instead. Plus the clunk every time the suspension compresses is annoying.
In short, if you have an Explorer (not Mountaineer) and you are looking for a deep, mean tone without a lot of noise, this kit is a decent one to consider.