[paulirish]

They're not a panacea but they are an enormous upgrade from your 1x raster.

I liken this situation a lot to adapting a site to multiple devices with media queries. Does it help? Oh hell yes. Would a unique mobile site be better? Probably, if you had the time and resources to commit to it.

Of course custom raster will look better, but I don't think it's realistic for most teams. If you didn't handcraft the pixels of your favicon you're not a candidate. Vector solves much of the problem and will continue to, even when new densities come out.