Here are some facts on LPG (Australian specs)

Calorific value---50 000 kj/kg(petrol around
47 000 kj/kg)
prefered a/f ratio 15.7:1
boiling point-- -42deg cel
relative density as compared to air--- 1.5
octane rating---- up to 110
prefered comp ratio 10.5:1
expansion ratio from liquid to vapour-- 270 times
spontaneous combustion occurs-- 430-440c
Here are the pros for lpg

cost saving in fuel used
cost saving in engine repair(less wear)
cleaner burning(less combustion chamber deposits)
reduced emissions--co,hc and lead
no pilferage of lpg
safer than petrol(if system is leak free)
smoother power from engine
easier starting (hot or cold)
As far as cylinders go, yes the carbon steel ones are quite heavy but there are still stainless steel ones around that
reduce the wait heaps.
more power with a turbo charged engine is achieved
thru being able to match the a/f ratio thru to high boost (up to 30+ lbs boost)without the aid of any electrics(computer
etc)
we have done it on several vehicles and stunned the guy at the dyno by recording very high horsepower outputs.
enough of the classroom hehehehe
cheers
Dave