I have actually just last week got rid of a 320i M Sport Highline Coupe after 1.5 years of ownership.
In day to day driving they aren't bad, but they are definitely on the slow side. Pulling out of a T junction for example requires you to really floor it to get up to pace with the traffic, as does overtaking on the motorway, you have to really boot it.
Economy wise I'd say I was averaging about mid 20s with a mix of town and dual carriageway driving, with the odd run out.
Mine had the engine management light come on intermittently for about a year, took it to various BMW specialists, none of which could fathom the issue. whenever this happened the engine would run roughly, with lumpy acceleration. The timing chain also needed replacing at 50000 miles at a cost of over £1k.
The clutch is the same as most BMWs, crap. It has no feel to it whatsoever, and changing from 1st to 2nd makes the car judder for a second before the acceleration kicks in, especially noticeable in slow moving traffic. I believe this is largely due to the clutch delay valve that all BMWs have which can be removed if you desire.
Mine also produced a huge cloud of vapour from the exhaust on cold mornings, a lot more so than other cars, even sat behind a 640i once that kicked out not even a fraction of the smoke mine did. A bit embarrassing.
The interior spec isn't bad, but now rather dated. I also found it near impossible to find a comfortable position in the driver's seat.
All being said I'd much rather have went for the 6 cylinder version.