Her short-period Q blind makes for great mid-game dueling.
Her W reveal is good for finding enemies out of position, and securing teamfight control.
Her E isn't much of a gap closer (unless you're in valor mode), but it is a strong attack because it leads to some horrendous burst (attack a marked target, E, attack is a strong combo)
Her R is ridiculous. You receive very powerful attack speed and movement speed steroids, but you become melee. Fortunately, you still have your blind, and your gapcloser actually becomes a gap closer. When you go crow form, you're Yi with a blind (but no reset on kill).
Unfortunately, outside of her mark, her base damages on her abilities feel really weak. Her Q is mostly utility and isn't worth maxing after playing her, her W is just a reveal, her E might be more worth maxing but I haven't tried it and don't know the damage per level. If the base damage on E is higher than her Q, it's probably better to leave Q as a utility move and max out your early-game burst as much as possible by vulnerable->E->vulnerable.
But in all honesty, she's definitely a carry. The closest carry to her I can think of is Vayne, because Vayne probably has the highest dueling potential beside her. Since her abilities have low base damage and she has such incredible steroids (movement speed on R and attack speed on W), she NEEDS items to be effective.
My 2 cents.