I made DP watch all of Buffy for the first time fairly recently 
It took him a while but he loved it by the end.
I forgot how wonderful James Marsters is as Spike. I listen to his Dresden Files audiobooks so am now so accustomed to hearing him speak with Harry's American accent it really threw me to see him in early episodes as Spike again before his accent settled a bit! If you'd have told me ten/fifteen years ago that I'd prefer hearing him speak without Spike's voice I'd have thought you were mad.
The acting, particularly from SMG, JM and ASH is just sublime all the way through.
ASH has some of the hardest scenes to do well. Just paragraphs of exposition and yet it seems so effortless and is still entertaining when Giles gives basically all of the background on the big bad that you need. That must have been so difficult to pull off so consistently well. He seems to draw focus without really doing anything. Just wiping his glasses or similar, yet for some reason your eye follows him. Wow.
SMG just is Buffy. She's said in interviews she found it really hard to separate herself from Buffy and I think you can really tell. The Body episode, particularly the first part where she's in the house making calls...I just weep uncontrollably every time. She's a hugely underrated actress IMO.
JM is another underrated actor. Particularly in the later seasons, the way he plays Spike is just masterful (excuse the pun ). It's the constantly shifting micro expressions that take his performance from good to amazing every time I've seen it, and he plays all of the (many!) different facets of Spike incredibly well. The scene in the church with the cross...
I could talk about Buffy all day! Enjoy the rewatch!