Pekka, we may have read the same anecdotal information, I've read that some of the more exotic magnetic materials will permanently lose their magnetism between 80°-100°C.
I've got a Prusea I3 clone and a rebranded Wanhao D6, and I mainly print PLA. I ALWAYS used blue painters on the I3 and had no problems with the part no sticking to the tape. When I got the D6 clone about 18 months ago, I naturally used blue painters tape on it too.
On both printers, with larger parts I would have trouble with the corners of the part pulling up off the bed. The part wasn't pulling off the tape, but the part WAS pulling the tape off the bed. Eventually I determined that the D6 clone came with a print bed that I could print directly onto. I normally print PLA, and it would not stick to the print bed. I got an Elmer's glue stick, and we were off to the races. The only glue stick I could find was the purple Elmer's kid safe version. Since I started using the glue stick the only times I've had trouble with bed adhesion on the D6 print bed are when I didn't refresh the glue often enough, or I tried printing on an area of the bed where I hadn't applied the glue. I'm seriously considering getting a replacement D6 print surface for the I3's print bed, it's less than $20 and they're the same size.
I use the sharpened metal putty knife that came with the D6 clone to remove the parts. One thing that I've found about removing the part - as the heated bed cools it will be easier to remove the part.
Don
Don