Of course I would watch, just a few suggestions to make a Spike spinoff work.
1) The character of Spike works well playing off other people, so the cast would need to be ensemble of course. My suggestion would be to turn it into a buddy action comedy with Andrew as Spike's sidekick of sorts.
2) No more CA, I think for Spike to work on his own series he would need to inhabit new never before seen Buffy landmarks. My personal choice would be for him to be on the road alot.
3) Like many other said, a Spike spinoff should not be "Angel" part 2. He needs a different purpose, because he seriously isn't a champion of Angel's stature, he doesn't want to be. Spike is a character who has always been lead by his heart, even when he didn't have his soul. I say add a little twisted romance to it, either have it be a new woman or Spike on the road searching for Buffy. But still, it needs to be based upon how passion doesn't make someone think most of the time. Spike would never think things through like Angel, he would just "do it" and that difference would make the difference in the theme and feel of the show.
4) A different kind of enemy this time around, Spike shouldn't be fighting your run of the mill vampires or demons. He needs to fight maybe something new, like a group of bigot humans hunting down decent demons or sorceres or something of that sort.