Pros:

-Miku
-high-poly model
-glows in the dark
-FP arms
-icons (lobby, in-game, incap)

Cons:

-textures are kind of flat [i.e. skin and clothes are just shiny and blank, not much detail on their surface]. It's a minor thing, and you'd probably only notice if you're picky (-0.5)
-no custom voice (no points deducted. Though one day I would love to see someone translate all the survivor lines into Japanese and dub them with Vocaloid)

Conclusion:

I've been waiting for XenoAisam's Miku Append to get ported to L4D2, and it's here! Great mod, no negatives that are worth passing this up. If you're a Miku fan, get it. Like, now.