Actually the tank in the boot is very discreet and hidden under a false shelf.
I`ve only had the gas for about 6-7 months, 3 months to go before MOT.
Cost of LPG is about half petrol but the mpg is a little lower. I am averaging 8.5p per mile which is about 20mpg. That is for all mileage but the main miles on the motorway usually cost nearer 7.5p per mile. That sounds crap but I drive with a very heavy foot and usually at about 80-90mph for most of the time. If I drive at 50-60 and gently I can get about 28mpg (this is a 2.5 V6). The best petrol consumption I ever had was 12p per mile driving very carefully at 70mph on a long run. If I drove on petrol like I do on gas I would be getting about the same 20mpg.
What I have found is I can drive with absolutely no regard for the cost of fuel and enjoy myself instead of worrying about the pennies all the time. The power is down to about 80% of petrol but it needs a tune at the moment, 90% is nearer the usual, but who cares about full power when all you are doing is cruising the motorway for hours. LPG is also not quite as smooth as gas but that all depends on the LPG brain chip mapping - I`m off to a specialist soon to get mine tuned up a little better - should get better mpg after that too.
Oh yeah, and when the pesky BMW 323s (or even the upstart 318s!!! - in your dreams matey!) want to play with you then you press the little microswitch and the petrol is back in an instant allowing you to smear them into a dot in your rear view mirror
OK, so bad points are: the tank, which takes up space; the cost, about £1500 - £1800 depending on how many cylinders you have. The good news is that if you do lots of miles it keeps your engine nice and clean (no carbon in the oil to send it black) and it has a very high octane rating so you have no chance of detonation, and its cheap!