I think they have to go with your style. I agree the face is important, but so is the hair. To me leather trousers are for women who take care to look stylish and that takes work. Not high maintenance exactly, just not someone who puts comfort at the top of their list.
Leather is usually shiny-ish which adds pounds, so slim is good. Bike leathers apart, don't do double leather, just as it's best to avoid double denim (on top and bottom) unless you know you can pull it off.
Check out a website called Pollyvore for ideas on how to put together a look for wearing leather trousers.
Personally unless I was size 12 or less (fat chance), I would walk away.