Leather surfaces in new cars are a nice uniform dull finish. After years of use, areas most contacted such as seat bottoms and the steering wheel become shiny, which looks like crap. In addition, light-colored leather seats show dirt a lot worse than dark ones, and both our vehicles have beige-ish seats. The only review of a leather cleaner I bothered to read on this forum pictured black seats, which really wouldn't show dirt to begin with.
I tried some leather cleaners, including fancy Meguire's leather dressing stuff, and all it did was make the seats shinier, slippery and smell funny.
We also had a few rust spots in the leather from putting wet skis inside the vehicle. So I decided to commit the blasphemy of using Fantastik on a rust spot. To my surprise, while every other cleaner had no effect on the rust spot, Fantastik almost entirely removed it.
So I tried Fantastik on the dirty and shiny areas of the leather seats. It removed at least 90% of the soiling and restored most of the dull finish. Not knowing if I had insulted the leather gods, I finished by applying the Meguire's junk. Which restored some of the shine, but reduced my guilt.