I'll put myself in the camp of "a bit annoyed by this". Today, charging at the Vaughan Supercharger in Canada, I got a notification that stated "Charging Complete. Idle fees are now accruing." However, when I went to move the vehicle (it took me 1 minute to get there, according to Teslaf, but 3 minutes by my own calculations) way less than 1/2 of the 20 stalls were occupied.
The feature definitely needs a notification that provides more warning - not the 15m earlier "your car is almost done" when it was at nothing like 80% full.
Also, note a workaround: assuming you, like I, are not in the habit of charging your vehicle to 100%, simply upping the charge limit will give you plenty of time to get to your vehicle - easily another 20-30m of charge time to get from 80% full to 100% full. It seems much better to move your vehicle if you don't need the charge, but 0.50/minute is pretty steep when the supercharger you use is at least a 10m walk from anything at all (Barrie Mapleview, anyone?).