I ran a 2008 320 CDI Sport estate for 4 years and over 40k miles. I bought it at 7 years old, with approx 68k on the clock and full main dealer service history.

Real MPG: Average (50/50) 30 to 31 MPG (just the driver). Long run at m-way speeds (London-Cornwall) 43 MPG (4 occupants + luggage). In town - low to mid 20s.

The good - the car is spacious, has cavernous boot though rear seats have less room than I expected. It is well specced and cheap to buy (but not to own) for what it is. Interior is sturdy and had no rattles after 11 years and well over 100k miles.
Good ground clearance (better than some crossovers).
It's not fast, but has good amount of torque which means it is quite nippy.
Gearbox is ok, though being adaptive can get annoying at times.
Crucially - no rust to be found anywhere after 11 years, so much better than W210.
Engine is good and pulls strong. It should last for literally hundreds of thousands of miles if services correctly. Shame you'll spend a fortune on other bits in that time.

The meh - Command system. Whoever designed it was fascinated by jets and their MFDs. Problem is - it's a poor design in a car. Too many buttons that may or may not do something depending on where you are in the menus. Also, some things are set from command screen, some from trip computer in the cluster using the steering wheel. r****** design.
CD player is hopeless. CD changer (the car had both for some reason) is excellent.
Satnav doesn't take full UK post codes but does show traffic info (not 100% reliable though)
Leather is hard and thick, which means it is resilient to tears but not scratches.
Rear electric blind is really loud.
Parking sensors can randomly chirp up for no reason when stationary in traffic.
Steering wheel way too big.

The bad - Reliability. Between 80 and 100k I had to replace pretty much entire front suspension (wishbones, control arms etc.) all the glow plugs and glow plug relay. From day 1 turbo actuator was playing up (the car would "jump" instead of accelerating) though this went away on its own, though I've set aside money to fix this in the future. Sport rims are r******. They cracked twice on the inside going over humps and one buckled after hitting a pot hole.
The drive - disappointing, uninspiring. On M-way it's fine, it glides and devours miles, but it is not a driver's car. It doesn't like corners and wants to throw its weight out of a bend. Hard suspension, harder than 3 series M-Sport, it just crashes about.
No cupholders - seriously MB? You've installed ash trays/mini bins but in the front there isn't a single cup holder? What were you thinking?
Long overhand in the back - I've sc***ed the back underside a few times going through a dip (high, low, high) while getting off a driveway, because the rear ends soooo far behind the rear wheels.