Been super lucky, pulled Herc and Edison, but I knew Edison kinda sucked. Luckily I just pulled Caster Gil, so is Edison worth keeping?
Personally I'd keep him if he's your first copy, even if you might never use him. SRs tend to have higher event bonuses than 1-3*s (if the Servant does have a bonus to begin with). So he might still be useful at the backline if an event comes along where Edison gives a drop bonus or something.
I personally like Edison. I got him currently at NP4 and i grailed him to 90. I might grail him to 100 one day if i get NP5. i find his 2nd (Mass Production A) and third skill (Concept Improvement A+) quite useful.
Edison is niche, but not useless. Combined with servants like Jekyll or Gilgamesh, he can actually have pretty significant impacts. The ability to increase the level of overcharge for a servant's noble phantasms adds for some interesting combinations.
Edison actually got another strengthening quest to his Shapeshift skill in Japan today (now it can also increase the defense of an ally plus also reduces skill cooldown by 1). It'll take 2 years, but I'd hold on to him if you haven't gotten Tamamo yet.
Well that's interesting to know. So...
Tamamo and Edison to reduce cooldown, Edison boosts Tamamo's overcharge, and Tamamo's overcharged NP boosts NP Gauge.
