From different BMW forums, the "most" ignition switch failures were reported by people who used a heavy chain key attached to the car key (i.e. lots of different other keys & objects). It seems that the weight attached to the car key, will lead to a premature failure of the ignition switch. If you don't use a big, heavy keychain, then there is something else going on.

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.