She's extremely strong. What's weak is her existence vs champs who have serious poke before arrival.
I just tried her vs a Bot MF (the one champ who doesn't behave much differently if a bot or a player is playing it....), and omfg.... early game.... even late game..... there were times where Qiyana just blew up as soon as you started the fight. But..... if you managed to W (Stone) > E her, Q, her toward a wall, and then R, OMFG the damage.
Qiyana can currently potentially 2 shot. It just takes setting up the plays.
Her clear is overloaded
Her CC for an assassin is unreal (I think she's more of a skirmisher, but Riot wants to go with assassin label to give her a stealth)
Her W can dash over a wall better than Riven or Nidalee can, and is on short cooldown.
She has one of the best multi-camp kits for jungling in the game.
All three forms of her Ult do the wall explode thing, and her Q is designed to abuse it.
Neither her E nor W actually require on target clicking (just proximity).
Learn that in duels, you need to get your target in between you and a wall, and there is almost nothing they can do to stop dying.
(That's going to take a lot of practice, because as I said, she's the kind of champ who will often blow up before she actually sets off her kit)
I think honestly a lot of people will do better with her if they build some tankiness. Like Sylas, she suffers from spammability. She can compensate loss of per hit damage to add some tankiness to do more.