Funnily enough I am currently driving a 2007 Porsche Boxster and thinking about a 1 series. My Boxster is just the base with a 5MT but it feels and sounds fantastic and the shift action is sublime. The downsides would be interior noise level and a very busy ride because you feel absolutely everything on the road.
I actually drove a 2010 135i this week that had 29k miles on it and was impressed with the amount of power and the smoothness of it's delivery. The car was loaded with M Sport Package, Premium Package, Cold Weather Package, etc. and had the 6MT so while it was fun it didn't shift as good as the Porsche and the road feel, while great, wasn't quite at the Porsche level either.
I will say that folks who say that the Sport Package 1 series with the run flat tires ride badly probably should not test drive a Porsche. After the Porsche, the 135i felt a little cushy to me!
I'm conflicted because the Boxster is a great vehicle and a very well built one also. I would, however, like a quieter, more refined ride without giving up performance so that's why I'm looking around and that 135i was a little rocketship.