On a gas car it actually works out very well. I am still considering it for my car. Advantages are:
1) you provide additional fuel without taxing your normal fuel setup via pressurized propane bottle
2) the added octane is like running race gas all the time when spraying further fighting detonation.
3) the fuel/air charge is further cooled due to propane being very cold when exiting the lines right along with the nitrous.

All that adds up to bigger shots without pulling as much timing and less worry about detonation. The thing I dont like is the idea of a propane leak in the engine bay...doh!!!!