I have just got my high up, cant really give you a "real life" MPG yet, but boot space is pretty good. the high up has a double level boot, ive hidden a blanket, towel, boots, water proofs, spare oil, bat and ball in the under level (i do outdoors stuff lots!) and could easily fit a weeks shopping of food for 2-3 people in the boot ontop of that.
only niggly bits as far as im concerned so far is the fact the seats dont go back to origional position when you get into the back, so my way of fixing this is getting back seat passenegers to only get in and out of passenger door