Had this one serviced a few weeks ago (about 68000) previously done at about 25000. No problems at all. Service cost £30.
The previous one was serviced after about 40000, and was ready when I sold the car after about another 38000. No problems.
First one did about 110000 on gas in two and a half years and wasn't a very good system at first, had it back three times before it was sorted - it was the first of it's type, I found out later. It was OK after that.
When the time comes to get another vehicle, it will definitely be another petrol and I will fit LPG to it.
The cost of this conversion was £1800, so as I've now done 70000 (based on todays local gas price of 46p/litre and petrol at 105p/litre) it would have cost about £6950 in fuel.
Plus the conversion cost makes about £8750. That's based on 21mpg, it does more on a run but it's a reasonable average.
The same car doing 24mpg on petrol would cost about £13900 on petrol.
Steffs' GV would work out about £8950 as tractor fuel is about £1.07/litre (£4.86/gallon) again based on 70000 miles.

Hmm, you probably think, not much difference, but remember that you can buy a V6 much cheaper than a tractor - I paid £13650 for mine with 57 miles on instead of £22000 for a new one. That saved me over £8300. Even though the road tax is £380 a year, it's still saving me money and I will keep the Santa at least another couple of years.
I've got a nice car that will tow more than other models with different engines and very smooth to drive, service costs are cheaper for a petrol than tractors, so for me, it's worth it.
You have to do high mileage to make it worth while though. 70000 miles in 29 months, and I've really cut down this year - only done about 15000 so far. It osts about the same to run as a 'normal sized' family saloon like a Ford Mundaneo or Vauxhall Vextra.