Yeah, just going to the dog park, the gym, then making it to my parents' in time for Thanksgiving would have required some planning in a ~70 mile BEV, or a lengthy L2 pitstop. No problem with the REX. I used the REX about 5-10% of the time, but for me, it is totally worth it since the i3 is my only car and I don't like the idea of the "battery camel" approach. Even with its limitations, the REX has got me through a few unexpected turns like a girlfriend moving to San Antonio (I live in Austin) without much sacrifice. Renting a car for trips like that is a non-starter and even if BMW would have a solid "flexible mobility" program, the dealers would have a no-dog policy, which wouldn't work for me. Plus, it is nice to be able to hop in your car and go, without making phone calls, scheduling, dealing with availability, etc.
I'd prefer a Mr. Fusion, but the REX really adds a lot to the i3, imho.