Many times when I heard Xander say something funny or sarcastic, especially in the early years, it felt to me like Joss Whedon himself was speaking to us through Nick Brendon's voice. Yet somehow, Nick made those Jossisms his own, and Xander developed as his own comedic and sometimes bitingly observant entity. I think Brendon is underrated as an actor--delivering those lines, some of those no doubt being Joss's favorites, must not have been easy. His delivery was fantastic overall, and something I consider irreplaceable about the series.
On to sadder
news: I hope Nick gets the treatment he desperately needs. Alcoholism is a horrible, horrible disease.