I actually made the switch and I'm not regretting it one bit. As a matter in fact, I don't think that you will find many to say otherwise.
To address what
T_flare was saying earlier:
I have put almost 8000 miles on the car since I bought it in June as my commute is quite long.
I do Geneva to Lausanne 3 times a week, which brings me to at least 300 miles a week with the car.
So far, it's proving to be an excellent everyday car. I haven't tracked it yet, but I plan to.
Anyway, it's comfortable for the commute and makes me look forward each morning and evening when I get to drive it.
And then there are times where I get to drive it on the twisty bits and you really see what it's capable of.
It's a very different beast to my 435d. It feels more raw, more planted and gives you better feedback on what the car is doing.
When you're in throttle mode Sport+, the throttle response is amazing. It just feels instantaneous.
When you accelerate with the DCT in S3 (most aggressive mode) and upshift, it feels like you're riding an uninterrupted wave of power
Oddly enough, in this car, my wife doesn't get sick anymore when I drive more ... spiritedly
I put it to the better suspension.
As a side note, what I didn't expect was the attention that it gets on the road, in a way that my 435 never drew. I've gotten waves, thumbs up and photos taken of my car from other people on the road.
That's not to say that it's a flawless car, but to me, it's by far the best one that I've owned to date.
The only downside compared to my old one (435d) would be the fuel consumption.
But I think against a 435i, the difference in consumption is lower.
I also didn't get to see the cost over time yet either, so that's something to think about.
Insurance wise, it actually lowered from my old one
I'd like to end by addressing
5ilver-5amurai 's point.
The next gen 3 series will debut next year, but the next gen M3/M4 will only start production ~2021.
That's a lot of time to wait for a "new" one.