The show went majorly downhill for me all the way back when Demming showed up. They started sacrificing Castle the character to turn the show into Beckett!
When it started into having the characters change for the plot is when I think it started downhill. That is also when Castle started turning into a bumbling caricature of what he was before too. He used to know a ton of off the wall tidbits and "Knew a guy" from past research for his book, then he became a Beckett supercop groupie, and was reduced to spit takes and reaction shots while panting after Beckett.
And yes I am quite a bit butthurt over the whole thing, I mean I really did like the show back then, I still enjoyed some of the eps and some of the flashes of what the characters were from years ago but they just got worse and worse. Watch any of season 1 or 2 up to Demming and you can see the difference to then and now. The only episodes where Castle was anything close to those early ones was the bomb flashback episode for the 100th and the stuff with Alexis missing. Well I guess 3xk too. Bleh. I am glad it is over now, I hope they all find better work than these last couple years were.