One word: sound. The cabin is filled with an incredibly sporty sound of the straight six which is quite unlike the sound of a V6. Makes me grin everyone I make a decent acceleration. The noise level (of annoying noise) is very low. Tires (205/55/16) hum somewhat and wind noise comes into play in speeds in excess of 130 km/h.
The car is equipped with OEM sport suspension. It is 15mm lower than the standard, but ground clearance is still better than in most other German marques. The suspension is decently firm and becomes excellent when traveling in speeds above 150 km/h. Doesn't make your teeth clatter in the city either.
Automatic transmission takes some performance out of the small six. The transmission is better in 325 or 330 as they have loads more usable torque. It is still a decently quick car in normal traffic conditions, but you do need to use the throttle. I use the Steptronic hardly ever as it isn't like in a manual; even if you have the second gear engaged the throttle response is as slow as in D or SD-positions. The revs climb up to the allowed ~6300 rpm in all modes (kick-down) and the next gear is engaged despite manual mode. Very handy for overtaking.
The cabin is not very spacious. There is a huge difference between a 5-series and a 3-series especially in the width of the cabin. The former owner of the car only had the car for a few months as his bottom never really became comfortable with the seats (a 62-year old man). My bottom got used to the seats after three days. These are standard seats with cloth. Word of advice for people buying a 3-series; get the sport seats. They are at least as comfortable, but they have more adjustments to make you feel really, really good...
The ergonomics of the car are otherwise excellent for the driver. My previous car was an E36 Compact so I felt instantly comfortable with the layout of the "cockpit". Everything works like someone really thought about it and tested it afterwards. The ambiance in the cabin is different than in any other car. That's probably one reason BMW owners are so loyal to the brand. Not everyone likes the BMW cabin, mind you.
The back seats are comfortable, but the space is limited. My SO and I are not very tall so the rear leg space is ample, but not so behind someone taller than 180 cm.
The boot has ample space for the stuff of two people on holiday, but not for mom's, dad's, two kids' and granny's. For that you would buy a 5-series Touring anyway.
The running costs are somewhat high because of the six cylinder engine, but mostly due to the automatic. I get a mileage of around 10l/100 km (~28mls/gallon UK). This goes up about 20% in the city traffic and down about 20% in the longer journeys. The cost of insurance is not much different than that of my previous car (1.6l engine). The tires do cost somewhat more because they are 16" instead of the 15" in the 316i.