Um... Perhaps I should change the title of this thread now. Don't want to unnecessarily upset any K-5 owners!
Just wanted to round off by sharing my experience with the guy who sold me the converter. I mailed him and first he said he'd take it back and refund me. Half a day later he changed his mind and fed me an incredible amount of bullshit in barely understandable language!
I've also bought a lens from him, a K 28mm/3.5 that had a sticky aperture. He'd described it as in great working condition. When I complained about the sticky blades he said I had to push the white button on the lens to unstick the blades!
I couldn't believe my eyes. He was referring to the small white dome that serves as a guide for mounting the lens... I politely expressed my disbelief, but he insisted. After his fourth mail of this kind I just dropped the whole thing and left him feedback that will hopefully warn a few buyers off.
I got the stuff from him through the Swedish ebay, Tradera. So just a warning to any Swedes here, or any others buying on tradera.com. I do recommend that you avoid seller id "jorstorp". This guy seems out of his mind...