If you're expecting to seat people over 6' in the back seat, they will thank you to order the panoramic roof, which adds noticeable headroom (at a guess, 1-2", 3-4cm).
At that heights though, I find the Model S' headrests to be pretty useless
I wish Tesla could address that with adjustable rear headrests - they are too low for tall people.
As an Asian with short legs but taller torso, I find the back height cramped and I'm far shorter than 6'. My son, who's just under 6', really can't fit in the back seat. Even with the panoramic roof (which we regret not getting), the cross beam in the back is the issue if one wants to rest against the headrest.
In any case, I did recently test drive an i3. Things to watch for:
1) Glass distortion. I found the windscreen glass distortion to be really bothersome.
2) Poor regen level. The regen is not adjustable and I found it very weak, much harder to do one pedal driving in traffic.
3) Inferior interior. I found the interior to be much worse than Tesla's. Maybe to some people, arrays of plastic buttons looks upscale to them. It doesn't to me. The unfinished hemp body panels also look... well, unfinished. The UI interface design of their infotainment/control systems is like working with Symbian from the 90's. The expanse of space in front before the windscreen is very Dodge Intrepid-like and I didn't like it. The doors seem fussy. The luggage space is minuscule, although anything is better than Tesla's cup holder and rear storage situation.
4) Driving was unspectacular in every way. Nothing particularly wrong, nothing particularly right, at least not in the short ~10 mile circuit I had to try it on.
5) Ugly. I was hoping it would look better in person. It doesn't in most angles. I'm actually ok with the look in the first picture (more of a side picture), and the look from the rear/rear quarters is ok. Front quarters (as one goes further towards the front from the first pic) to front look is pretty much as ugly as I had feared.
It is likely I need to buy a urban/short distance commuter car before Tesla's Gen 3 so I went in genuinely shopping. I'm not sure I'm willing to buy this car, but the other choices aren't very good either. I'm hoping I feel better about the Mercedes B-class, even though it isn't offered in my state this year.