The i3 is no Tesla, but it's a great little car. It's a fun car and I enjoy zipping around in it. I did NOT get the REX. To me, that's a big waste of money and it is for amateurs. I think it costs around $4200 and it adds 300lbs of weight. The extra weight adds about 1-second to the 0-60 time and drops the electric-only range from 83 miles to 71 miles (22kWh pack). It's a very light car and that's where it gets it's range and performance so adding the REX really has an impact.
From what I've heard, people who get the REX (and a lot do) never even use it. To me, it makes no sense to lug-around a 300lb motor that is there just in case when there are charging stations and electricity out there almost everywhere.