I had 2.1.1 and found that this plugin will complain about: undefined method `asset_file_path'.
So just a note to users that it's either time to upgrade Rails (my guess is at least 2.2.x) or put that method into the plugin's asset_tag_helper.rb:

class ActionView::Base
   def asset_file_path(path)
      File.join(ASSETS_DIR, path.split('?').first)
   end
end

Also, for those poor souls running development on cygwin: you will need to change compressor.rb compress method by adding cygpath -m, e.g.

def compress(...)
  cmd = "java -jar `cygpath -m #{self.class.path}` `cygpath -m #{input_path}`
  if output_path
    cmd << " -o `cygpath -m #{output_path}`"
   ...
end