Interesting that our experience was exactly the opposite. The xdrive35i felt heavy and ponderous with a lot of throttle lag. We assumed this was because the demo vehicle only had a few miles on it (200 miles ish). The xDrive28i felt much better and responded almost instantaneously to the throttle although my wife complains that all the BMWs we have had (and we have had 17 of them) have had the same issue (a lot of 'have's and 'had's in that sentence, sorry). Stomp on the throttle, there is a slight pause and then it takes off like a bat out of hell. This problem also afflicted the MB E500 we owned. You quickly learn to deal with it.
Originally Posted by KP49
Luckily we have a six so there is little hesitation, I barely made it out of the parking lot to test drive a 28i and turned around and laughed at the salesman. My wife liked the X1 but even she noticed the lag/hesitation right away with the 4cyl. Took a 35i for a ride and it was a lot better.