For me, the character has clearly gone downhill.
Throughout pretimeskip he was a solid character. Attractive fighting style, entertaining battles, balanced running gag (never been the biggest fan of it, though), badass moments (Mr. Prince, the hunter, asking Enel for a light...) and an interesting role as an emotional yet empathetic leader; his moment of complicity with Usopp during his Water 7-Enies Lobby conflict, when they spoke about what being a superhuman means, remains one of his best moments and one of the most sincere interactions between two Straw Hats.
But postimeskip... What can I say? Overused nosebleed gag, significant lack of badass moments, no relevant fight for ten years (and Oda could have given him one, easily) and the Vinsmokes. The Vinsmokes may easily be one of the worst ideas in this whole series. Where should I start?
-Feeling like a cheap soap opera, the only way Oda found to make us hate the siblings was to turn them into the perfect Sanji antithesis. They waste food, they beat women, they are classist...! And what is even worse: Oda retconned Sanji character, turning him into some kind of messiah of kindness, only to stress the contrast with his sociopathic family. Sanji, the person who didn't care about Wiper being severely injured, or would have left the kids in Punk Hazard behind, or advised on leaving the minks untreated. That Sanji, for some reason, ended up risking his actual friends lives to save a dictatorship like his biological father's (apparently because that's what Zeff, a man nicknamed Red Leg because he was so brutal his legs ended up all bloody when fighting, would have wanted).
-The siblings are said to lack emotions yet they clearly display them throughout Whole Cake arc; this is because Oda simply followed the easiest, yet worst approach to the concept of an emotionless character, which in its laziest execution equals a very bad person. And on a personal note, I was expecting a "Reservoir dogs"-like group (following Sanji's inspiration and how they were labelled by Pekoms as a family of killers) yet we were given a Super Sentai scientific kingdom. At least Oda kept the thematic color idea...
-Sanji's past adds next to nothing to his character (the only interesting part was how it could enhance the words towards Usopp I referred before) and was quite boring to me. I don't have much to add about this.
-The Raid Suit. My God, the damn Raid Suit. Horrible design, doesn't fit Sanji's aesthetics at all and virtually kills everything interesting about the contrast between what Sanji was able to achieve by himself (flying, armament haki, producing fire) and what his siblings were granted just for the sake of their father's evil science (flying boots, exoskeleton and suit protection, their supernatural ability). The interesting part about Sanji being a superhuman by himself was his sentiments, his raw power, his fiery heart... in comparison to the cold war machines who had their emotions erased. By giving Sanji a suit, not only you erase part of his timeskip progress (what's the point of skywalk now?) but destroy all of what I've exposed about the contrast between he and his siblings.
Currently I don't enjoy Sanji and I doubt it will get better since Oda's treatment of his characters has been overall poor for years now.