That's right the performance is poor as the engine drops to 100 bhp when running on gas (which is methane, not propane). However, the fuel consumption is very good and it is as cheap as driving around in a Fiat Puno. The engine tear is also less as it is running on gas. You get less particles with gas that grinds the engine. In general I'm satisfied with the car, especially with the all rising oil prices. Unfortunately BMW stopped making CNG cars and when on the fuel cell bandwagon instead.
If you're going to buy a new CNG car then you should look more into the Volvo V70 or S80. Mercedes also recently came out with their E200 NGT. These cars have much better performance, equal to petrol cars. The E200 is 160 bhp and only consumes about 6kg/100km which is very good.
BMW 518g touring (e34) -96.