In WoD I've been pretty indifferent about Demonic Servitude. It wasn't exactly interesting, but it didn't really matter since it was hardly ever used for DPS in raiding apart from them few months in BRF when Demon Bolt got a nerf.
After seeing the Warlock changes in Legion though, I've really grown to hate it. I just don't think it has a place anymore. I look at it and all I can see is a something that's taking up a talent slot.
Like people have mentioned; it's boring and disengaging, it removes our ONLY cooldown, it doesn't work with the sacrifice talent, it removes the fantasy of "each spec having one specificly-themed pet" which was mentioned at Blizzcon, thematically it doesn't make much sense for Affliction and Destruction anyway, it's thematically okay for Demo but removes the spec-specific Felguard so again doesn't make much sense and finally... it has no niche.
The game has come a long way to cater to the people that don't want to push themselves or take part in any competitive environment but a talent being purposefully included just for the lazy people that CBA to manage an extra button or two doesn't sit well for me.
If Demonic Servitude stays for that reason though, I'd only accept it if it was the worst talent in any kind of competitive gameplay. Let the people that want to be lazy have their exciting demon (that's not so exciting now that its permanent) and I'll be happy with choosing an interactive talent that makes me feel like I have to actually try in order to yield the results I see.