Conrad,
I'm pretty familiar with them. Installed one on my parents house a while back.
If you live in an area that gets a lot of hail, I know it costs more, but everyone around Oklahoma recommends spending the extra $$ and going with the heavier guage sheet metal. Less prone to hail damage/dents.
Usually, once the liner material has been laid on the roof, there's 1"x4" planking that's nailed/screwed on before the metal goes on... but not always. Most metal companies want this, and affects warranty.
Walking on this type of installation becomes more tricky, to avoid denting, since it's raised.
But, I will say, it goes on fast, and lasts.
And, as someone else stated, helps to externally fireproof the roof. Now, it doesn't help if a fire is internally started... But say a neighbor's house nearby catches on fire, or a grassfire happens, or flying embers from a fireplace happens to land on your metal roof... It helps.
But, as mentioned... it's a lot slicker to get on! Roof pitch can really make a difference.