I know 2 people with LPG.
1st has an older shape Range Rover and he said its great - he gets what would work out as 35mpg which is pretty good for a 4.4 litre petrol. He had it installed which cost £2k, but i dont know if he's done the miles to make it pay for itself over 5 yrs...
2nd mate had a mk1 Vectra which he had converted to LPG... he had it about 8 yrs, but he had frequent problems with the ECU - it kept screwing up and he had to take it to the garage all the time.
I'd be nervous about getting one myself to be honest though...