I think the people criticising it are either people who've never actually used it, or people who've seen cars with poorly installed kits and assume that it's the norm for LPG.
A high quality kit installed by a professional should not give you any problems. My dad's got a Jeep with the 5.7 Hemi engine which was converted at 10k miles, and is still running perfectly on LPG at 70k. He averages 17MPG on LPG, which gives him an equivalent 30MPG, as LPG costs 60-70ppl here. In Europe, it can be as low as 40/50ppl which is very useful on road trips.
The main issue surrounding LPG is simply that it's a cost saving measure, and is favoured by the kind of people who like to save money. Which means that they skimp on the kits, installation and maintenance, which leads to problems just as it would with any type of car. Get a car owned by the right kind of person, or pay for a good conversion yourself, and you'll be fine.