And yes, I'm the developer, so I am biased.
And no, it's not a plugin. It's a stand alone application.
You need to get your titles in over either PNG's with an EDL (that Premiere will link to without problems), or trough a QT with an alpha channel.
So the downside is that you cannot subtitle 'on the fly' while you're editing.
The upside is that using this tool, subtitling will be way faster, giving you tons of editing shortcuts to move text around, see timing issues etc.
And of course, you end up with a subtitle file that can easily be converted to any other format for re-purpose, or easy translation to another language.