OK, not ruined but it is certainly my current obsession.
It was kept under control while Andy and were working our way through the box sets (if under control equals watching two episodes a night) but after watching Sunnydale being swallowed up I was on the internet catching up and trying to find out whether the consensus was that BtVS had essentially gone downhill after Tabula Rasa when magic became a metaphor for addiction (it isn’t, it’s a power not an indulgence) and we had to watch Buffy and Spike boffing. I think the answer is yes, and, don’t get me started (not just now anyway) on what they did to Willow in Season seven.
There is so much! I am currently reading my way through The Chosen (I have linked to the description free episode list because the pictures on the home page contain a spoiler) which is a continuation of the series (I think there are others) and it is remarkable. I have just finished episode 03 of Season 9 and so far, the storylines, the dialogue, the humour, the drama and the guest appearances are spot on. It’s not perfect but then when was the “real” BtVS perfect (not after Tabula Rasa anyway!)?