Did a 300 mile road trip in the 128i today and I thought there might be interest in a few observations.
The car is very comfortable on the highway for the most part. The suspension is very well-composed (I'm sure the Bilstein/Eibach setup has a lot to do with this, but the basic character of the design seems sound), and it's nice and quite (70mph is 2500rpm in 6th, which is 500rpm lower than top gear cruise at the same speed in my 525i). The one negative compared to the E39 is the effect of the short wheelbase, which makes the car a bit more, um, lively, on rough patches of pavement. Jarring would be overstating it, but it's definitely noticeable.
The manual sport seats are astonishingly good. Once you figure out how to make them fit you properly, they're wonderful. I got out of the seat after 2.5 hours and felt no discomfort, no stiffness, nothing. These are absolutely worth holding out for if you're shopping one of these cars.
The power delivery of the N51 (and the N52 - they're virtually identical) is very different from my past BMW engines. Where the typical BMW inline engine likes to be up in the top third of the rev range to make best power, the N51 is much more linear. You don't need to downshift nearly as often, but you miss out on the fun of hitting the power band as you wind the engine up. The 6spd is really, really nice. Well-defined gates, short throws, very communicative.
Controls are typically BMW logical. Everything's where you expect it to be (the E39 was the same way, as are most BMWs without the iDrive setup or whatever they're calling it now). It is a small cabin. For two, it's perfect; more would be a bad idea unless you have no concept of or need for personal space. The trunk is pretty small too, but again it's not an issue if you consider it a two seater.
No big surprises, but maybe useful to someone shopping for one of these.