Now I have used this lens/camera some more, and have come to the following conclusion:
- outstanding bokeh for large oof areas/low contrast situations
- horrible bokeh when stopped down with strong light-sources in the background (Pentagons)
- bad bokeh when wide open with strong light-sources in the background (my inital finding) For small enlargements this seems to be of no practical relevans, but for big enlargements it does.

I am quite pleased overall. The outstanding bokeh that I sometimes get from this lens is the reason I bought it, and it enables to get the mood I was after - but I have to remember the bad performance under certain conditions...basically there is no point in taking let's a stopped down portait if there are strong lights in the background - the pentagons are just too distracting.