The custom heads (potentially unintended) feature is no longer working. Player skulls are still functional, but custom textures obtained using this* don't show textures.
Unintended or otherwise, this was an exceedingly useful feature to map makers and redstoners and it would be an immense shame to see it go.
In otherwords, please fix or the hugs are only hollow. :'(
WAI, unfortunately. Custom heads are not gone entirely, though. They just can no longer be loaded from arbitrary servers, in which the website you linked to falls under that category.