If it ended at 5 you would have never gotten to see Spike being the lead and the focus of the show and his path to redemption. Even in 5 he seemed largely comic relief, it was nice to see him grow beyond that as a character.
Spike being the lead of the show? Above Buffy?
Interesting comment but one I totally disagree with.
And it goes to not only what happened with BTVS itself but also a similar situation on ATS.
Cordelia was the second main character or in some eyes co-lead of ATS for her entity on the show however with her exit the show still continued on.
Now I'm not for one moment not going to say that Spike's story ended up taking precedence over the scoobies and the Buffy's in series six and seven (to the cost of the show IMO) BUT if JM had decided that series 7 was going to be his last and not SMG, does anyone really think that would have still been the end of the show? From my perspective I think JW would have continue to do the show without Spike because Buffy is/was the lead of the show and her mot being there effectively ends the show which happen.
As for this series five. Love it for the emotions of the story with the arrival of Dawn who I never had any major issues with. It's my 3rd or 4th favourite depending on my mood and certainly my 2nd or 3rd series/season finale. And IMO it connects so well with the series/season two finale.