Our deck is a combination of pressure treated wood, Trex and cedar - pressure-treated wood for the structure, Trex for the floor and railing, and cedar for the structure and roof of a gazebo. That combination was required because Trex doesn't have the structural strength required for structural work. The deck is now 14 years old.
My experience:
Trex is great, and generally lasts a very long time. But it is NOT maintenance-free. Mildew will collect on the surface where it is not exposed to direct sunlight, so it is necessary to scrub it down with a detergent/bleach solution once a year. And if you choose a strong color, it can fade over time. Also, because Trex is a soft material (recycled plastic and sawdust) that doesn't have much structural strength, the recommended joist spacing is 12" rather than the standard 16" - if you have 16" joists, you could see ripples develop over time. Finally, long runs can expand/contract with temperature leading to buckling (especially for things like fascia boards).
Cedar: it's ok, but it does weather over time, and to maintain appearance, you probably need to apply a stain initially and periodically thereafter. Periodic sanding may also be required. Also, at least in our area, we have 'carpenter wasps' that will burrow into the cedar to lay their eggs. The solution is to force a mixture of waterproof glue and sawdust into the hole, let it cure, and then sand smooth.
Both the Trex and cedar in our deck are holding up well after 14 years
