You pick her when you are confident the enemy team will let you farm all your camps in peace, i.e i wont pick this into an ivern, leesin, though i might pick it into an amummu, rengar, sej, ect ect. though she requires a VERY SPECIFIC RUNEPAGE BUILD! attackspeed reds +quints, scaling armor, scaling magic resist, 0/18/12 stormraiders + vetern scars. I will on occasion also use that rune page for WW if i feel that they have too much burst ap/ad. I go this runepage because it lets you focus on hp damage items like hydra instead of the armor versions you'd be forced to buy early otherwise.
excellent early/mid game champion with an ok late game, you pick him when you simply are not confident enough in any other jungler into the enemy team comp, he does ok vs everyone but not great, he is also an excellent enabler for your teammates when built for it. i run him attk speed reds, scaling hp yellows, scaling CDR blues, 2 attk speed quints + 1 scaling CDR quint. though this build assumes a certain level of confidence in your jungling and outplay potential along side ability to effectively use the CDR, its fine to just go the scaling shyv page or a more traditional jungler atk speed page if not confident.
a note on shyv, you should still make what i call symbolic ganks on her, yes you will likely fail miserably but thats not the point, the point is to not let the enemy laners think you just wont show up ever, you put the dragon born fear into them by doing it.