Since Item 3285 has came out on the PBE, I've built it on Katarina on every game I've played with her. Out of 16 games, I have won 13 games, going negative (16/17/10 KDA) in only one of them.
Now, in my honest opinion, I believe it will be a very strong item on her if it gets pushed to live in its current state. The build path synergizes with her qualities so well, and, conveniently, she does not need early mana sustain like most other AP assassins/mages, making it a very valuable item to rush.
* Case in point, Katarina *loves* movement speed. What better way to get it than through a 120 AP item that gives her enhanced wave-clear and short bursts of damage after every rotation of her abilities? Movement speed allows her to pick off badly positioned targets more easily and do it more safely. It also allows her to roam much faster. Katarina already has high base movement speed. With just 2 movement speed quints and 3 points in Fleet of Foot, she will be unkillable (by anyone that is not Vi or Lissandra-esque point-and-click CC, for example) unless she commits.
* Unlike Katarina, if someone like Ahri or LeBlanc were to rush Item 3285, they would not have the mana regeneration to stack up its passive as fast as Kat would. They would run out of mana pretty quickly spamming spells. For this reason, they and most other spammy AP mages have to build a Morellonomicon/Athene's first. Sustained AP mages like Cassiopeia and Karthus could fare well with the item, but they need an early Tear, so I think Katarina (maybe Kennen, but I don't play him) reigns supreme for the best user of Item 3285.
The last 5 games, I had been experimenting with 10% CDR in runes and building 40% CDR with Item 3285. With 40% CDR, Kat stacks the item passive VERY quickly. I transitioned from playing her as an assassin who goes in at one time to playing her as a skirmisher. She just has so many options you honestly can't go wrong. I'd like to see some people try the item and 40% CDR to see if it is indeed a generally strong build.
Yet another topic I'd like to bring up is whether it is an equal or better replacement for Deathfire Grasp. In my opinion, it far surpasses everything DFG stood for on Kat. Who needs to assassinate one target ASAP when instead you have even more AoE damage spread over more targets as well as the movement speed to facilitate that?