I have to say that overall, I've been left wanting with my sudden new passion for Buffyverse/Supernatural crossovers. Especially disappointing is that Dean/Xander don't seem to be hooking up nearly enough. And, when they do, they seem to already be involved or were involved and that totally misses the fun of them getting to know each other and then falling into bed together, which is where my passion really lies, I guess.
It's the same with Spike/Xander - it's the build up and the crescendo that really makes me fall in love with a story.
I did run across this story though that I did enjoy: Thirteen Days
Alas, there are no slashy elements (despite my finding it during a search for Xander/Dean) but it nicely combines Xander, Dawn, Sam and Dean in the same story. The plot is very obviously and heavily inspired by Dawn of the Dead (2004), which is as good a setting as any. It also has Xander/Buffy, which I'm not really a huge fan of, but since Buffy goes for help it doesn't get in the way of my story enjoyment. I also like the smattering of Sam/Dawn and wouldn't mind seeing more of these two paired up in future, too. Nicely, as well, Riley makes a guest appearance and isn't evil or psychotic.
So, I'm gonna go ahead a give this one a recommendation. I've decided against doing fanfic reviews as such (exception: Watchers), but if I were to rate this one, I'd give it a 3.75 out of 5.
And, since I'm using a new "fanfic recommendations" tag, let me add another... "Long Time Gone", which is truly an epically long story, but one that I have not stopped thinking about since I read it a few years ago. This would be a Spander story mostly, but there are also other pairings for the Scoobies, including Buffy/Angel, in case you can't stand that. But, it is quite good involving Xander leaving the gang behind post S7 and then returning to Sunnydale after his wife (who the Gang never knew) is killed by Drusilla. It's not perfect - some of the Xan/Spike scenes are a bit too romantic for my view of Spike and they fall into sex a bit too fast and too often for my personal liking (Xander's like a machine), but it is so well written and characterized that I just find myself loving the story: 4.25 out of 5.