It would be nice addition to Safari when released for free or with source.
Unfortunately it's a paid plugin, which basically duplicates poor solutions from other del.icio.us tools for Mac, do not introduce anything new (well, maybe except the fact being first one of it's kind to exist in form of plugin) and misses some basic functionality.
There is no tag suggestions, no autocompletion and all your tags are displayed as a single flat list (which is insanely long in my case). There is also no way to filter out tags or search your bookmarks without getting on del.icio.us website.
Short description: feature limited Pukka, which is available for Safari only.
Definitely not worth $15 at this stage, get Pukka or Webnote Happy instead.