I bought a Multipla in november for the christmas trips, carting family around etc.. and to see what they were really like. An X reg 1600 with 90000 miles and a snapped cam-belt. 1 low mileage bravo engine later and it runs like a dream. (what a way to use up holiday at the end of the year!!)
I drive a MG ZR diesel normally and the multipla is smoother, quieter and lighter to drive!!
Have done about 1000 miles in it so far and its great, running around and doing 70ish on the motorway it returns 33-35mpg (27-29 on the computer).
Push over 70 the aerodynamics with hit the mpg (not rich enough to find out how badly). Unladen it is more than quick enough and picks up from 20mph in 5th gear and does about 3600rpm at 70mph.
Its also nice to have a heater that warms up quickly in the current weather conditions.
With a full compliment of passengers (no luggage) it does slow it down a bit but if its infrequent I guess its not too much of a problem.
I havent tried a diesel but would personally consider it over the petrol due to the fuel costs, but the purchase price for the petrols is lower and if you prefer them it will balance out.