I think Viktor has strong early and late game already, due to his early farming and poking power and his late game AoE effects (Chaos Storm, Gravity Field) make him very strong in teamfights (the ratio on Chaos Storm alone is 2.3 per ability power; that's the same ratio as Crowstorm). However, I bought Viktor with the intention of being able to adapt to the match using his variety of cores. Unfortunately, out of three cores, I see myself using Death Core the most because of the ability power bonus and the boosted Death Ray ratio. To a lesser extent, I use Gravity Core because of the CDR bonus and the extended reach on Gravity Field, but I never see myself using Power Core because I find its health and health regeneration bonuses underwhelming.
About Power Core: Rylai's Crystal Scepter and Rod of Ages have health bonuses much more impressive than the mere 220 health, and the +6 regen per 5secs looks and feels very unimpressive, not just because mages rarely use health regeneration items, but also because +6 health regen is extremely small! Doran's Shield alone gives +8 regen, and many other items with health regen dwarf it noticeably.
Besides, even if it gave more health regen, why regen in the first place? Power Core almost feels like some kind of bizarre support item, with the subpar health bonus, health regen, and mediocre ability power bonus. the speed boost it gives to Power Transfer is definitely interesting, but the first thing I think of when I look at its stats is not "Tanky Mage Item". Items in that category are items like the aforementioned Rylai's and Rod of Ages, Zhonya's Hourglass, and Abyssal Scepter, items with much larger boosts that make you more sturdy in the face of immediate damage. Returning to my complain on health regen, I can understand why tanks and supports would want it: Lifesteal and spellvamp are useless due to their lack of damage output. A mage, by definition, deals large amounts of magic damage, so why not spellvamp? Why not some armor or magic resist?
On another note, I think everybody also agrees that the ability power bonuses on Gravity Core and Power Core are underwhelming, barely breaching 50 AP at level 18. Why are these bonuses so low, when many other ability power items give over 70 AP?