From http://lpgautoconversions.co.uk/faq/index.php:
Does LPG Autogas hurt the engine?
No. In fact quite the contrary. As the fuel is a gas it mixes better with air than petrol, therefore the combustion is much smoother, and also cleaner. The engine will last longer when using Autogas. Oil and spark plugs will also last longer.
Your performance is also not greatly effected, however, you will notice that your miles to the gallon will decrease slightly.
You get approximately 80 - 85% of the mileage that you normally get on petrol. For example - if 1 litre of petrol takes you 10 miles, then 1 litre of gas would do 8 - 8.5 miles. However, many converted cars do better than this.