Originally Posted by r3dbimmer89
Technically speaking, the N55 is more advanced than the N54...that engine is long gone once the 335is/135is/etc are out of prod.
Minor correction - the 135is uses the N55, it is the Z4 35i and 35is models that still use the N54 (in addition to the 335is as you say).
Your conclusion is spot on though; once the models that currently use the N54 go away so too will that engine. The B68 and S55 will be along soon to make people forget all about the N55 though. It won't be missed for long.