In my experience, LPG can be good, but there are many things to consider as above. Converting a Petrol to LPG or buying a diesel may be a close call, but some people (me for example) may simply prefer the quieter sound of a petrol engine, or have a sentimental attachement to a well maintained car or have other reasons to want the petrol.
The most cost effective way to run around on LPG is to buy a car that has already been converted at the expense of someone else, as long as it has an LPGA certificate (or is on the register) or your insurance co may not touch it. They usually transfer only a small value across when you sell, so its only worth doing on a car that is young enough, and will be used enough, to cover a significant number of miles. Cant be many LPG converted A2's on the market though....!
The (probably only) great thing about increasing fuel prices is that although the cost of LPG has doubled in the last 10 years, and now hovers around 70ppl, the cost of petrol has also gone up two fold, which means running an LPG car now, actually saves more fuel costs than ever before. But thats after its installed, you've lost your boot space, informed your insurance and ensured there are a good supply of LPG stations on your usual routes (which is likely as they're quite well spread now)...... You always get a little less power on LPG too, so factor in around 10% drop in MPG to your numbers. Also you will start and run the first few miles on petrol, so still need to buy some of the expensive stuff from time to time.
I've had quite a few V8 A8/S8's on gas, and it was economical motoring (relitively), mainly because I bought them pre converted at little extra cost. I've never personally considered converting a car though as I know I wouldnt keep it long enough to get the value back