I have. Trust me when I say it is worth every penny. Right now I have a VW Passat Syncro (4WD) 2.8 V6 which is LPG converted (was converted by previous owner). I had a problem with the car and it was off the road for almost a month. In between I drove a 2.5 V6 TDI Passat, 1.4 Polo, 1.8 Peugeout 406, 1.8 Vectra and believe me when I say this that I really appreciated and realised how much more economical my Passat is than all of those cars. Yes even more economical than the 1.4 Polo.
I have already worked out that the £1600 I spend on conversion is something I will save in the first year. The other thing you have got to remember is that when you do the conversion not all the money you spend on the conversion is gone. A big chunk of it turns into assett and you can recover it once you sell the car. So in effect you are only spending 500-1000 on the conversion which you will easily save in the first year alone.
Plus you will never fail an MOT over emissions and you will be harming the enviornment a lot less. :winky: