Word of warning: since the warm weather came and people started switching to summer wheels, police is out checking their Einigungserklaerung. I saw them a few nights in a row, but tonight they stopped me. Turns out that I'm not allowed to drive with the summer wheels (which are now on the car), even though I have an Einigungserklaerung, because it's not stamped by the STVA.
The policeman was very nice though and let me off without a fine and explained me that all I have to do is go to the STVA and get the erklaerung stamped and I'll be fine. Too bad the STVA is closed on Saturdays