So I have driven the same M4 twice in the last two days. There weren't any M3s on the lot, so to get a general feel for the S55 and the better suspension, it was M4 or no drive at all.
In full disclosure, the car had less than 100 miles on it, so the sales guys were very adamant on me keeping the car under 5K RPMs. I was able to do that for the entirety of both rides, but for one blast off the line where I got close to 6K in second gear.
First and foremost, remember that i went into the test drive not really knowing what to expect. The F8X Ms haven't been universally praised, unlike the 235, which has been repeatedly praised and adulated over...and which I haven't been very impressed with - so I tried to be as objective as possible.
The M4 had DCT and the Executive package, so HUD was present. No adaptive suspension.
Steering Feel: this is one place where I had felt that the 235 wasn't bad - the electric steering in the 2er is pretty good, and I haven't had many complaints. Until I drove the M4. The sharpest setting was satisfyingly heavy, while still feeling quick and easy to direct the front of the car. The M boffins clearly have put some time into this part of the car - it is much better than the 2er. Not to say that the 235 is lacking, the M4 was simply provided a more rewarding experience. I liked it a lot.
Brakes: this is another place in the 235 that I haven't had complaints with - and once again, the M4 was better in my opinion. The M4 I drove had the normal brakes, not the CCB option. There was, overall, a more secure pedal feel and more bite as you pressed down. It really felt like you could bring the car to a composed stop in a very big hurry. Nice upgrade over the 2.
DCT: This is a tougher comparison, as my 235 has the MT, but I have to admit...man this thing shifts fast and precise. In the most aggressive setting, it just pops through the gears, and it is quite nice to feel that from the driver's seat. There is also a nice little blip in the throttle when you are slowing down and in the most aggressive setting - which wasn't present in my DCT-equipped 135 from 2010. I was pleasantly surprised overall and this is one option I would definitely consider after getting some time with it.
Suspension: Far and away my biggest gripe with the 235 - aka the "squishy" moniker than i am now mocked with by my buddies.
The M4...ummm...doesn't suffer from a similar problem. It was nice and stiff, and felt very composed over both smooth and bumpy portions of the road. The test drive took me on flat, curvy, straight, highway, etc. - and there wasn't a situation in which the car didn't excel. The car just felt so planted - you could feel the weight of it, but at the same time, it felt tight - that's exactly how my buddy CHack described it, as he was in the car too. Very big difference here, and in a good way in my opinion. I was truly impressed with that - and if the M2 has a similar feel, it will be leaps and bounds over the 235.
Engine: this one has a bit of a caveat, that i mentioned above - I wasn't above to really push the car through each gear - I was able to go fast and feel how the car responded at speed (the grip is incredible by the way) but I am not 100% sure on the engine. It certainly
sounds better in the cabin, but I am not sure if that is merely active sound shenanigans at work or not. Definitely felt faster than the 235, but how much I cannot say. From 1-5k there is a significant difference, but I wasn't floored by the engine. Perhaps it comes alive further in the rev range - I haven't done enough research to know where the max HP and Torque come in. One major difference is how quickly the S55 revs. It just whips through the range and before you know it, you're in 4th gear and approaching silly speeds. I was really impressed with that aspect of the engine response. The 235 truly felt lazy afterwards. Is it manic, in the same way that my dad's Z4MC is? No - but again, I wasn't able to really rev it out either - my guess is that there is quite a bit left in the 5K+ part of the range.
Throttle: Again, another big difference here - generally speaking, both N55s I have had seemed to lack the same throttle response of my N54 - but the S55 outshines either one by a long way in my opinion. I am sure I would drive around in the most aggressive modes 99% of the time because it is so easy to get what you're looking for out of the gas pedal - the engine responds very quickly to slight or large inputs - which is nice. It seems like I just have to floor the 235 now to get it to move with any urgency. That gets old after a while, and reminded of it once again after getting back in my 235.
Final Thoughts: Wow...what a car - I really really think I would like the M3 after that demonstration. I think the 3 looks better and weighs about the same, plus it's a little cheaper. The car sounded better, had a much more capable suspension, unbelievable grip, outstanding brakes, and the first version of the X55 engine that I liked right off the bat. And that was just up to 5K RPMs. In short, the car is leaps and bounds above the 235, as it should be, being nearly 25K dollars more expensive. But I would skip out on most of the options - just DCT, 19s, and likely LEDs and Alpine White...which still leaves me in a conundrum - the F80/82 are awesome - but are they $400
more a month over what I am already paying awesome? I am not sure of that. $200, yes. $400 or more...I don't think so. As a result, I am going to seriously start looking for someone who will deal closer to the 7% below MSRP like David Aviles - if I can find that, I think I am going to do it...but we will see.