I looked into using it when I built my deck, but was going to cost about 4 times more than pressure treated. Good stuff though, never heard a bad thing about it, except for cost.
I just finished having a trex deck put in last fall. I don't recall the $/linear ft. off hand, but it is a bit more than getting wood. However, it is more than worth it in my opinion. Wood decks need to be scrubbed and restained every couple years, whereas the trex is pretty much maintenance free. I can look up my receipts later if you want pricing, but I would recommend it over traditional wood if at all possible.
Rough idea on $$ per sq ft.?
Cannot emphasize this enough. Trex has little bending resistance, so you will probably want to put your joists at 12" OC, but certainly no more than 16" OC. Also, do not make any portion of the substructure out of trex, use pressure treated lumber for that. You will also have to predrill and use screws rather than nails. And remember, for exterior use go with galvanized, but preferably stainless steel fasteners. Galvanized is cheaper but the galvanized coating can deteriorate and cause staining.IIRC It is not a stiff as wood, so you might plan on putting your joists on closer centres...