Running a 4.6 stage II tune.
She runs sweeter on LPG - smoother with more low down grunt, but better power on petrol at high RPM. No problems at all in 17,000 miles on the v8, and 100's of thousands on other cars (including a 235bhp Saab Turbo)
If you do fit a kit, make sure the underlying engine is 100%. LPG is harder to ignite, so it'll find any problems with your ignition. It also runs a little hotter - so make sure your cooling system is in good nick. Choose your installer carefully - there are a lot of cowboys about - go for reccomendations - I use HKL gaspower in Brum.
You do lose a little power (The LPG takes up more volume in the air/fuel mix than petrol, so you lose some volumetric efficiency) and economy (LPG has a lower calorific value - less energy per pint!) But as it has a higher octane, you can run a higher compression ratio (if you are rebuilding an engine specifically for LPG) and more ignition advance. But watch out when you switch back to petrol - it might pink/detonate.
Also, fit as big a tank as you can - once you are running on LPG, you never want to switch to petrol.
I have twin 36 litre sill tanks and two 40 litre tanks where the petrol tank used to be. Range is about 330 to 400 miles depending on journey and driving style. Have a small (70 mile) petrol tank in the rear wing. So have optimised it for LPG - and tweaked the petrol injection to run rich in order to avoid the pinking of the advanced ignition.
The engine runs a lot cleaner on LPG - plugs come out really clean, so does the oil.
Have fun.