My 4.4 is LPG converted - it was done when I bought it, but I test drove a LOT of 4.4's before I bought it and it drives no different. In fact as you can turn off LPG and run on petrol by pressing a button next to the gear stick, i often put it in petrol mode to test and its impossible to tell a difference, so no loss in "fun".

Only you can decide if its worth it, based on how long you're keeping the car. Mine was no more expensive for having been converted already so its a bonus for me. I get my LPG for 62p a litre and petrol is £1.30+ here, so I am paying less than 50% of the cost of petrol, but LPG is around 10-15% less efficient than petrol (so 13.5mpg instead of 15mpg) so my petrol bill is about half of what it would be.

If you save £100 on petrol costs a month (based on how much you're currently putting in), it will take you between 12-18 months to get your money back (depending on what system you go for - don't just go for the cheapest), plus the LPG service will be about £100 per year.

If you are going LPG, you could run a LPG 4.4 for not much more than a LPG 3.0 (because even though it drinks more fuel, the extra fuel costs much less so the difference in running cost is less noticeable). Just a thought.