I went out do dinner last night & came home with something sticky on the front of my bag. I tried plain water & it didn't work. It kind of pulled one of the threads a bit too.
I have never tried it on my bags but that "goo gone" stuff is very effective removing gunk. Its like an oily citrus based product. Maybe do a test patch and see how it effects the leather first.
woops! i dont know if they are just legally "covering their butts" but they say: Do not use on silk, leather, suede or rubber..... so maybe my suggestion isnt so good..... sorry.