I had an LPG Vectra Estate, factory fitted (Well, technically after market as Vauxhall didn't fit them themselves but outsourced it) but anyway. While the cost was obviously very good (LPG was 35p a per at the time) I had all sorts of problems with it. From trying to find someone local to service it, to engine problems, LPG cutting over to petrol at random times, blocked valves, all sorts. In the end I just gave up and traded it in for the Saab 93 chug chug.
I'm sure the technology has improved since then, but I'd be really reluctant to convert anything after my experiences. You've also got to do a lot of miles to make the cost of conversion pay off.