I used to work in the CNG industry, so i got a couple tips...
CNG conversions are not nearly as efficient engines as fully dedicated ones. Cause of the difference in octane ratings (your engine was designed for gas and CNG is the equiv. of 120 octane (if i remember correctly). Anyway, its less efficient. If you want more information, you can contact lauren @ raymundo engineering and she has lots of information on that type of stuff - but she has a dedicated NGV rig.
But you're absolutely right on the fuel and they aren't bad (the conversions) . Plus, you can get a slow fill dispenser and install it at your home and then just fill overnight. No more gas stations. If you get a conversion, you will have access to almost all NGV filling stations throughout the country. I think lauren had a map of everywhere she can fuel. Most of them are like company fleet maintenance yards, but they work!! The advantage of the conversion is that you can use gas when you need to, and then switch to cng when you can find a spot. Who knows, it might be perfect.
If you call lauren, tell her I said Hi.