I've been playing a lot of Viktor lately, and while I believe he's usable, he needs some small buffs to truly be competitive.
For starters, unless I can build straight AP or Augment Death, caster minions have a tendency to just barely survive a death ray, which is quite a disadvantage in bot lane compared to casters like Ahri and Malz who can clear waves in a single spell with beefier builds.
Secondly, while most caster's ultimates cooldowns lower each level, Viktor's does not. The long cooldown really hurts him in a setting where team fights and skirmishes are constantly occurring.
Lastly, I really think his passive should have a second tier upgrade. This is just hypothetical completely unbalanced or mathed out numbers here, but say the second upgrade costs 2k gold, and it upgrades his ultimate by lowering the cooldown a flat amount (say 20 seconds), as well as an additional benefit based on your previous augment. For example, Augment Power could provide you a shield for 30% of the damage done by the storm lasting until the storm dissipates, Gravity could cause the storm to silence for three seconds, and Death would let the storm last seven seconds longer. Perhaps there would be a more balanced way to do it, but I really think improving Viktor's passive would be the most interesting way to bring him up to par.