While since I've posted in here, but the long and short of it is I sold the Smart back in March.
Basically, it did the job. I'd wanted to own one since they were new, especially since owning a first-gen Insight for a while, and then being intrigued as to whether the Smart would be any more economical.
Well, after 19-20k miles, it was... basically the same as the Insight's economy. 60.2mpg US, to the 59.7mpg US I got in the Insight.
The difference was that the Smart was less pleasant to drive, frankly. This is no great surprise (tall short diesel semi-auto vs. low longer hybrid manual) but it did start to grate on me after a while. The Insight is a firm riding car and can get swept around in the slipstream of trucks and faster cars a bit, but magnify those characteristics and you have the Smart. It just got a bit wearing after a while, more so than the low performance or the semi-auto gearbox, though the latter, while you get used to it, still does something spectacularly dumb now and then which also wears pretty thin.
Good stuff? Park it anywhere, decently comfortable over longer distances which you'd not expect, cabin space/storage are excellent, and I still like the design of it inside and out.
Will I miss it? No! Next question... (longer version: I've now ticked that box and have no desire to go back.)
I've replaced it with this:
Obviously not as economical, but at more than 33mpg US it's currently comfortably beating its EPA rating without even trying. Super consistent tank to tank too, whereas the Smart was all over the place depending on seemingly endless factors, a tiny percentage extra throttle, temperature, weather, how windy it was etc. And I don't really need to explain how vastly better to drive the MX-5 is...