I had a 3.2 Touareg converted to LPG about six months ago. Cardiff Autogas did a great job of lifting the rear floor so that the tank was covered and installing the controller in the cabin, really quite neat. The system is almost seamless, automatically switches to LPG once the right temperature is reached, is just as smooth and I can't discern a lack of power.

I estimate that the £2000 cost wil be recovered in around 12-18 months as I get the equivalent of around 30MPG now based on the revised LPG cost.

The main drawbacks are the fact that there is no road tax rebate, the loss of spare wheel well and lack of availability of LPG but otherwise I am very pleased. I really don't understand why there aren't more vehicles available or converted, even small cars have a huge jump in economy. I had a very old Volvo S40 during the conversion and this seemed to get around 70 MPG equivalent.
If you have an old car you can extend the life and have great economy with LPG - seems a shame that most people dismiss this as a solution.