Thanks Xeins
The i3 REx I had was of course a UK model, so others may differ.
The REx function/motor could be switched off fully, but when the battery depleted to a very low level (about 5 miles I think, when I noticed it), it turned on by itself. It can also be made to maintain charge at anything below 75% battery. All this via the I-drive control.
It is quiet, but you can hear it. It is louder in the rear of the car, but no more than any petrol car - if not quieter than most.
I cannot comment on fuel usage as I barely used it. I planned on buying the BEV car, which I did.
Only you can decide if it is worth it, and how much range you require, and how much peace of mind you need. The trade off is a heavier car (I HATE that), that is slower and slightly worse handling, with higher maintenance and running costs.
The i3 does a great job of calculating available range as you drive, and whether you can reach your destination, so I had little need for REx.
I would never buy any i3 if it were my only car though, even the REx.
It is a tough decision though, for all of us first time electric car buyers.
Good luck!
Jas