Dear lord, how many forums have you asked the same questions on?
Excluding regs and empty (yes, air does weigh something!) a standard alu core fibre wrap 68/4500 weighs about 1.1kg, a Fuel SL weighs 650g. I believe the Fuel tanks were released last year - there were certainly adverts for them all over the PSP webcast. Personally I like a light setup, so for me the weight saving of the Fuel outweighs (badumm tish!) the downsides (expensive, only a 10 year lifespan rather than 15).
One other thing to note is that as the Fuel is smaller than most 68/4500 tanks, some bottle covers will be loose. The Exalt tank sleeves fit fine though.