some of pop's best songs make use of nothing but add9 chords. good examples of this are message on a bottle by the police and satellite by dave matthews band.

the nine adds a sort of "open" sound to the chord. if you'd like to explore this kind of sound, i'd start listening to tori amos or kaki king.

as far as your own stuff, i wouldnt rely too heavily on the nine to give your tune the biting edge, but if you're looking to to add some flare or some personality, i suggest I-7add9 or IV-(add9). these two chords play vital cadential roles in contemporary music.

check it out!

hope this helped, get back to me if you want some more tips straight from berklee.