Some time earlier this year when we were working on this years lot of municipality mergers I promised that I would produce some kind of analysis of the postal code tagging since it was likely that the merger work had broken it. Since early May the new release of the postal code boundaries is available and I've done some initial work see http://qa.poole.ch/ch-plz/.
While the information is useful (disambiguation of places with the same names for example) I don't believe importing* the polygons into OSM makes any sense at all, they are at the same time too close and too different from the administrative boundaries making maintenance a nightmare and just asking for unintentional damage. I would think that the best solution would be to add the postal code to the house addresses in places where we have multiple post codes in use.
Any other opinions?
Simon
* while the data is freely usable, if we do decide to import we would need to clarify the attribution requirements first.