I had a spin with Vel'Koz in the PBE and the early verdict is in, what a dumb broken champion. Just the way that Riot likes to design new champions these days I guess.
Five abilities that each do damage, including a passive that does true damage, what a surprise that he's a high damage monster! Oh sure, everything he has is a "skill" shot but we're not talking about Brand here, we're not talking about 700 range slow as fuck projectiles that can't penetrate the creep line. Vel'Koz's spells are 900+ range that go through everything. His W has 2 charges, because hey Vi is such a fun champion to go up against so he can cast W twice in 2 seconds. Face-rolling on the keyboard = outplayed. His E is a bit easy to dodge, but again puts the pressure on his enemies to dodge everything or die instantly and lose the game. His Q is his staple because of a low cooldown but it's weird since it doesn't go through creeps and splits off at 90 degrees, I predict no one will care about it. All his abilities get lower cooldown as you level them, naturally, and his autoattacks are tentacle slaps.
And finally his ult, oh god his ult, it's hilariously broken. 1500 range laser that goes through EVERYTHING for 2.5 secs and that has such a dumb high base damage that a full laser almost kills a squishy, without any AP whatsoever. And it's not like he puts himself "in danger", his ult has twice the range of a Brand ult. Hell it takes 4 seconds just to walk from the edge of his ult to where Vel'Koz is if you're some meaty bruiser, better hope you have 2 gap closers to interrupt him. Let's keep comparing his ultimate to other champs, slightly lower damage than a Fiddlesticks ultimate in the same 2.5 seconds, a very good hit box so it's essentially AOE, no channeling time and all from the comfort of your own home. All without any AP, in fact AP doesn't even matter that much when he's all about landing and stacking as many abilities as fast as possible and triggering his passive. I can see him built as an AP bruiser, 40% cooldowns, survivability and still wreck teamfights. I wonder how much he can slow when you combine his 70% Q slow and Rylai's. Vel'Koz overall just like Yasuo and Jinx IMO puts such heavy pressure on his enemies to dodge everything, remember his cooldowns and not die instantly that he will be the same terror as they are in soloque.
In short, he looks fun as hell if I played mid and really annoying to go up against. I just hope the next champion is a jungler so I can enjoy some of this.