I sometimes use it, especially before triathlon (because of salt and everything) or the running bit (again, salt) or if I didn't do a long ride in a while, am going on one, and feel like I could use it just in case. You just use a bit - a dip - and rub a thin layer where the saddle and your bum touches. Don't slather it all around.
If one bike I'm fine and the other is a torture device, probably different saddles?