Although knighterrant will have to give us review of the precleaner, ...
A very long time later, here are my comments.
Using the Turtlewax Black Pre Wax Cleaner on most of my C350's bodywork made no difference that I could see. I only used it on small sections at a time, but it was very easy to apply and polish off. But because I've always washed the car using the two-bucket method and used lots of different cloths when polishing, it's not yet suffering from any swirls so I haven't been able to assess the Pre-Wax effectiveness there.
But what I was eventually able to see today was how good it is at hiding tiny chips. When cleaning the car this morning I spotted a couple of tiny chips close together in the paintwork on one of the doors, each less than a mm long and clearly caused by a flying stone
. So it was out with the Black Pre-Wax to give it a go. It may be that I need to build up a few layers in the chips, but it hasn't hidden them in one go. They're far less visible, now being grey instead of white, and probably nobody else would see them. But I can! I tried to photograph before and after, but couldn't take anything that meant much. A couple of tiny dots on plain black aren't all that informative.
So far then, my observation based on its use on reasonably new unmarked paintwork, is that it's good but not wonderful. It may be effective on very light scratches. Hopefully it'll be a long time before I'm able to report on how good it is on those!
Anyway, here's a shot of how the car is now. I didn't take any care over the background but it does show quite well what Magnetite Black looks like (better than the previous photos that I've taken in the sunshine).