@Greg:
First and foremost, there's no reason to 'hate' on Sanji. That's absurd.
Neither should anyone shield him from the criticism he deserves because of how he's been handled.
Again, there are people who are bigger Sanji fans than you can ever imagine and even they admit he's been handled poorly nor has he shined as he had in the past.
It's okay to accept that and realize, Oda's been doing this intentionally and he's going to make up for it.
I sorta see it that way. The way I see it (in a more serious summary compared to that heavily jokey Sanji biography I couldn't help but to write), Oda initially just loved having fun putting Sanji's quirks to the test post-Enies Lobby (from the poster to Duval and everything in-between) but is some way planned a certain downward path of increasing misfortune with the Kamabakka Kingdom as a deliberate nadir for him in regards to his hopes and dreams. Post-TS has focused a lot on Sanji's "post-trauma" early on in which the whiplash from hell to heaven was so severe it took both a physical and mental toll–only that Oda preferred to keep it more or less lighthearted. I think that Dressrosa, while still retaining those Post-TS characteristics, is a transitional period of Sanji bouncing back closer to his old self.
In addition, I agree that the decision to have him humbled multiple times in combat is deliberate--the primary exception being Fishman Island which was necessary to show that Sanji had something positive to show in his horrific experience.
I have big hopes for this upcoming "Sanji Saga" since I don't think it will just be bringing back "early Sanji" but build upon his PSTD in new directions.
It's sort of the way Oda has treated Usopp. While Usopp never slid from the same heights as Sanji, Usopp has very often been given extremely embarrassing moments that would later be built on to either built him up or further tear down his confidence. I recall a similar outcry around the time Usopp quit the Straw Hats in which Usopp HAD to make a strong showing and that Usopp had to redeem himself. Well, long story short, he really didn't. For the most part he shined a few times in spite of repeated embarrassment and humiliation, realized he was essentially a big sack of hot air and showed his commitment to the crew simply by admitting it and accepting it. Also, despite having possibly one of the most significant post-TS power-ups of the crew he has still been a very confused person, not sure if he's really cut out to be a big-shot or just a regular guy way out of his depth.
Usopp's recent escapade in Dressrosa lampshaded this duality between the fabricating his own heroics and actually becoming one in his own way. The result was a more decisive admission and realization that he is a bullshit artist in addition to being more willing to take on more suicidal situations with both eyes open. Yet he is still reliant on gimmicks and dumb luck to see him through, further perpetuating future lies, boasts, and cold feet. The whole "God Usop"p thing is clearly set up to put Usopp though a similar a potentially more disastrous farce in the future.
While I'm making this sound pretty seriously, Oda is very adept in keeping these themes and seemingly painful transitions very light. Regarding both Usopp and Sanji and a few other SHs, often times the serious undertones would set up a very silly and comic situations. Oda really does like deflating heroics to either tongue-in-cheek or outright farcical proportions when he feels he can get away with it, especially with the SHs. I mean even the crux of Sanji's fall from grace is completely ridiculous. So while we may get a serious rebound or "depth" to Sanji in the future, I expect Oda to poke some fun along the way. Also, I think part of it may be that the comedic situations Sanji had been put through jsut weren't some people's cup of tea. Personally, I got good laughs out of most of it, but I can see that it may have worn thin to others over the years.
Back to Sanji, Oda is clearly knocking him down several pegs in order to build him back up at some point, again in agreement. I'm still in the camp that thinks he has actually been more interesting going on this roller-coaster ride of personality shifting and silly behavior. He's certainly one of the least "static" of the Straw Hats (for better or worse), in relation to the larger scope of the entire Straw Hat adventure. I think Franky may be on a similar route of partial character deconstruction if the Senor Pink experience is built upon in a later arc, but it's way too soon to tell.