We are using our i3 year round, including snowy weather as well as trips to the hardware store and pumpkin patch. I have years of experience with BMW OEM cargo liners in our former 2005 330i ZHP and in our 2007 BMW X3 3.0si.
Both OEM trunk liners function the same as the Weather Tech, protecting the cargo floor from all the stuff hauled in them. The real difference is how well Weather Tech liner fits and the quality of compound they use. They simply just fit.
You can see it in the photos where the Weather Tech tucks into the corners and wedges itself in there without buckling. The OEM trunk liners tend to slide around and their less grippy surface lets stuff slide around more.
Weather Tech says they used 3D lasers to measure the trunk of the i3. This requires the actual vehicle to be able to make their liners and takes about a month from measurement to production. The Trunk Liner has a lipped edge that will keep anything that gets spilled from coming in contact with your i3’s trunk which is literally right over the i3’s electric motor.
Another nice aspect is how grippy the trunk liner is when you set stuff on it. There is an optional Cargo Containment System for $34.95 that helps hold grocery bags and such. We have not tested it but looks to work as advertised.
WeatherTech Trunk Liners are available in black, tan and grey. The custom fit Weather Tech cost $109 and comes with a 3-year guarantee.