I am selling my M135i now...the B58s are pricey and and x35s seem to hold their value and price better since the x40is are out. Granted, they are both good motors, the B58 (like all generationally advanced motors) are more solid than the last.
But the N55 is a motor with 5 common failure items that 3 are oil leaks...and of the 5 I did not experience the waterpump failure or vanos failure. The Oil filter housing, Sump gasket and Tappet cover, I did all of those as preventative maintenance and the car has been solid as the day I bought it, So historyof the x35 is really important as well...they all driven hard but like all, well maintained and mechanical sympathy leads to a reliable driver.
Feel free to contact me for any other details if you need, I have a full record of all mods and maintenance i have done on my car since 83000km till now.