If you do a lot of mileage then yes its worth it, otherwise no.
If you're doing starship mileage, maybe, yea - but even so there's still lots of negatives. The average user will never see any benefits.
A quick case study (not for you B&W, sure you're aware, just for those interested):
18,250 miles a year on LPG, at 30mpg (assuming 65ppl, lower economy due to LPG installation) : £1,795.19
18,250 miles a year on petrol, at 35mpg (assuming 116ppl) : £2,722.38
Difference: £927
Cost of fitting a decent 6-cylinder multipoint system: £1,500
Time to recoup initial outlay: 1.6 years.
......so you'd have to drive the car for over a year and a half, before you even started to break even - and that's negating the cost of any replacement vapourisers, assuming LPG gets the same tax breaks, keeping the system serviced and so on.
Plus there's the real bugbear of limited range (although obviously more, larger, tanks can rectify that but then there's added weight) and the fact that it is
still quite hard to find LPG on a casual basis, and if you do a lot of pumps tend to be out of order. Also potentially weaker performance although that does vary.
If you're going to buy it brand new, a decent top-of-the-line system, maybe - but you'll be driving that rig for a looooooong time in most instances to recoup, or maybe even save, a little bit of money. Won't add £1,500 quid to the value of your vehicle, either - it'll just be another worn out motorway wreck with a few hundred quid tagged on because it's got LPG on it......
People seem to always rave about it but I think they've either very limited experience with the kit or have never actually done the sums
Second hand systems seem well worth steering clear of as well......we had a big long thread on RR about it recently and every single person had nothing but trouble with cars with 'working' LPG systems fitted to them.