The difference in the engine noise was dramatic to me. At idle, it's much louder, especially if you're in drive-thru line or somewhere where there is anything to the sides to bounce sound back to you. I'm not sure it's much louder driving at speed but the engine note is much different. It always sounded like it was out of tune to me. I hated it. Accelerating also often sounded like there was metal grinding on metal. The noise was a big issue for me. I never felt happy driving the car because of the sound.
Getting diesel was rarely a problem around where I live, but when it was it was annoying. Traveling outside of my area for example, was not fun. If I could locate a car diesel pump, it wasn't a problem, but often I was left with truck pumps, and I inevitably ended back up in the car with diesel all over my shoes and the smell just added to the stress.
The steering always felt heavy and sluggish to me. I even over-inflated the tires
by 2PSI to try to make up for it. It was great when you are driving on the highway or pushing the engine, but slow turns and whatnot just felt off. The heavy steering did not give me any greater sense of the road than I had in my previous X3 with active steering. It could be that the 18" wheels contributed to this but I'm very happy I'm getting the active steering on the 35i. Loved it on my X3.
Driving in D mode also felt sluggish, even with the extra torque. Driving in DS mode, though, was great. Really showed with the engine could do. In DS, everything felt smoother, faster, and more appropriate for the engine. So, I drove mostly in DS, but that took away some of the mileage advantage. The 8-speed in the 35i does a better job, I think, but I will probably miss some of the power in DS mode.
The constant DEF warning with a 1000 miles left was really annoying. I drive about 1000 miles a month or less, so that's a month's worth of constant reminders that don't go away until you click them away.