Finally spotted one of those Opel GT's the other day. Actually, my wife did. She followed a yellow one on a B-road for a few miles. There have been a couple of times when I thought I saw one out of the corner of my eye going the opposite direction on the autobahn, but this was a definite sighting.

Still waiting for them to come to rental car fleets (if ever) so I can take one to Nurburgring.