My i3 is the Rex version with the 94Ah battery. The range extender gives about 90 miles of use on the 2 gallon tank on top of the c135 miles of electric power.
Mine's also the 94Ah REx, official NEDC test gave 142.9 miles, I think WLTP was slightly better, in the real world best I get is 127.4 & I live in a very hilly area & like to make full use of potential 'acceleration'

Thanks guys, really interesting. I can definitely see an i3 on the shortlist in a year or two. It's interesting that BMW have now dropped the REx version, apparently people were finding the generator unnecessary in real-world use.