I have a few times, but not for many years. The main reason why they suggest it can be used on leather is due to the way automotive leather is coated with urethane to provide durability, so in which case you are treating it like any other vinyl or plastic surface. There are exceptions to that depending on car company, but for the most part, its P/U coated leather.
The best thing about 303 on "leather" is it dries to a matte finish, so no sickly gloss. However, despite the finish being dry to touch, 303 does make the seat base quite slick/slippery, which is why I didn't continue using it.