Assassin with %damage in jungle does not suffer as badly as jungle assassins who don't when dealing with bruisers and tanks. her W is usable as instant cc, a 65% slow with no warning can make or break a gank, if you can get an easy charm on a low mobility or over extended lane then obviously do it as hard cc screws folks days up, but its entirely acceptable to use her Q to break stealth then cast W before it lands to apply a fat slow with a long duration. Their best bet is to flash it,,, and hey you still have the W debuff growing into a charm waiting so your up a SS and not even down your CC. Also... its not hard to get her W as a charm off when your ganking, teammates with any CC make it pretty dam easy and most folks when they see that charm icon are going to try to run rather then stay and fight their lane to the death for the 1-1.5 seconds of not being cced by their laner so your lane gets that much more time to get their damage out retaliated.
E range is short, but if your between them and safety... they have to walk by you and into its range... so pretty easy to hit folks with it on ganks. Scales really well off items, its what, 2 or 3rd highest potential damage on any assassins basic ability. Your incorrect on her q and its damage, 330 damage with 1.95ap scaling level 5. Each point you put in Q is worth 50 damage, yes... that is her best case usage. Its not hard to pull off in ganks thou. W as a slow sets it up, W as a charm makes it dam near impossible to not hit best usage, if your able to E your set to follow that with Q and its best case usage as well.
So, play her when your want a mid game carry on your team, shes a real threat to back lines after 2 items and can snowball rapidly if she gets any momentum and destroy multiple lanes days.
Course shes also hard to play, requires good judgment and proper usage of skills, and like many assassins is feast or famine + if 2 lanes lose hard and fast your gonna suffer as much or more then other item dependent damage junglers.