Have I? Yes. Do I continue to? No.
I have done it before, with success, without causing any type of damage. I don't use the method any longer however, especially because other less potentially dangerous techniques can work, and work safely. I don't want to worry about potential delamination (urethane coating separating), stretch marks, and spot damage on my car or anyone else's.
Here's an example of recent work:
'83 Ferrari Mondial with less than 20k miles and the leather already had some hardcore wear and "cracking" spots (these areas were where there is even more build up, which causes more wear and build up, which causes more wear and build up, etc). Especially look past the driver's seat and look at the passanger seat bottom area.
I don't have a true "after" on the driver's seat
but know it was much improved. Here's an after of the passanger seat which wasn't as bad, but was still quite bad for her miles, that was turned around to like-new condition:
Here's a "near after" of the driver's seat as I was still applying an intermediate step:
Mind you this was two and a half hours of work with two guys, but it shows what's possible. Products used were Leather Master Strong Cleaner, Vital, and Protection Cream. Strong Cleaner was used with a toothbrush (hence the 2.5 time frame to complete), so every square inch of surface was thoroughly cleaned with high attention to detail.
For those more interested in Leatherique, here's a 50/50 shot of a Lincoln we did a couple years back with Rejuvenator Oil:
We applied the oil after a light vacuum to remove debris (always light vacuum before leather cleaning), applied the oil by hand (while gear gloves to not make a mess of things) by rubbing it into the surface, allowed to sit a couple of hours, and then began removing it using diluted woolite (20:1). You can also remove with Prestine Clean.
Doing work on vehicles that are in this shape takes much longer than leather care on a decently maintained car, so keep that in mind. On my own car, I give a thorough cleaning of the driver's seat one day, and then the passenger the other. This means I don't get too tired or worn out working on my car. I enjoy it more, and I'm more likely to keep up this type of work.
As you see, you can effectively care for leather using low risk products: a key in detailing your pricey vehicle.
Says the guy working for / in bed with Swissvax... didn't see that coming