I've used Friends a fair bit in EA. Typically, if you use it on an NPC, and you are still in sight when the duration wears off, there is a good chance they will turn red and attack you. Note that Animal Friendship has the same effect, but on animals.
If you use it on a companion, they don't attack you but your reputation with that character is lowered.
I haven't tried Charm Person, but a major difference is you can use it in combat to turn the target non-hostile to you.