Sweet Tour, worth the money!
What was out of anyone's control: We had bad luck and booked our tour on one of the coldest days of recent history (down to -14° C), so we couldn't take the Skodas - it just too cold. Definitely not our guide's fault. Instead we got to do the tour with a VW Caravan from 92.
Having said that, this tour was amazing and I'd recommend it to everyone! Our guide Peter knew a lot about the city, gave the tour a personal touch with his stories and (for this kind of tour not unimportant) drove us safely around town, in spite of the weather conditions.
I specifically like how Peter is interested in ruins and abandoned buildings (I am too) and knows where to find them. They are getting fewer, sadly.
I can only imagine how much cooler it would be with warm weather and in the retro cars. Seems I've to take the tour again some time ;)
If you get the chance, take the tour. Period.