Right...you won't find molasses in Guinness.
I've used in on occasion in both Porter and Stout but in very small amounts...just a couple ounces will be apparent in the flavor. But how much is right for you is something only you can decide. For my tatses, a little goes a long way (and how much or how little will also depend in part on what type of molasses you use)
Molasses can definitely add some depth and complexity. When you get into higher quantities, it can start to add some rum-like notes.
When I do use molasses though, it tends to be in stronger, long aged brews. That's just a personal preference.