Drove it into work this morning, and a bit yesterday evening.
First impressions are mixed.
As a car, having had my Z4 only for a month, I could probably drive the X4 quicker round a track. That's mainly because it just allows you to go all in, almost anytime/anywhere without the slightest hint of effort on my part. In the Z4 I would have to bring a spare pair of brown pants to do that!
In comfort mode it is very insulated, and just goes along. You still have the same power on tap, but it is late in responding to your power demands, which is fine as it makes for a more relaxing experience. You can still end up in 3 digit speeds quite easily and not be able to tell, had it not been for the HUD (which is awesome by the way).
In sport mode everything is kicked up a notch. The noise, the response, the steering and suspension go into battle mode. It is a very savage sounding engine, but you don't get the same kick up the behind as you do in the Z4 with its DCT gearbox. It is very flat round corners at speed, with very little body roll; very surprised at how flat it is for a car of its stature.
Overall, the driving dynamics are fantastic in both modes and deliver what I expected they would.
The cabin is good and relatively luxurious and the iDrive system much better than the one in my 11 plate Z4
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Where it's let down is the lack of safety features that are now standard on hatchbacks. There's no adaptive cruise control, nor lane keep assist, nothing basically. For a car that is meant to be upwards of £60,000, this is a let down. It's an SUV so would expect these things to be standard in what is supposed to be a family car.
The cabin could also be a bit better in terms of quality. There's certain trims (e.g. A-pillar) that are not necessarily in the immediate eyeline of the driver that feel cheap and can be pushed to reveal a huge gap without effort.