I've had the Powershot2000 system on my truck for over a year, and am impressed with the amount of additional power it provides. The installation is relatively easy, once you determine the tank size/location, etc. There's a true 80hp increase on a dyno. The cost of propane itself is well under $.01/mile, so the expense of operation isn't too bad.
The downside is if you have it set too high. Serious damage can occur if safe limits are exceeded. Bent rods and cracked pistons are a few of the potential dangers.
If you decide to install a system, let me know, and I can walk you through the initial "dial in" procedures.