@Revenant compared Melange to heroine - I would go step back and compare it to coca leaves and cocaine:
Fremen are born exposed to the Spice - its in their food, in the air - but in very basic, polluted form - this would be akin to the traditional use of the coca leaves by people living in the Andes - it helps them to fight the height sickness, provides vitamins, reduces pain, fights off asthma and increases longevity (another similarity to Spice is that it was also used in religion ceremonies). Similarly the Melange helps Freemen to adapt to the horrible conditions on Dune.
Navigators use the pure stuff that is pumped in the air they breathe. On top of that, if you believe the prequels, the Guild doesn't take just the first guy from the street and turns him into navigator - they candidates have first pass a series of university-level exams proving their intelligence and their body has to adapt itself to the Spice filled atmosphere (which causes their mutation).
Now, the @OP says that I cannot even remember anything being mentioned about [Fremenn] being longer-lived or healthier - please notice that those guys - basically a bunch of poorly armed tribesman that are able to take down elite Sardaukar army and later they are the core of Paul's army conquering and wiping whole words.