You should treat 9-nine- as a tetralogy instead of a standalone VN. True, it can be played as a standalone game, but it would seem to be very lacking, as its story and plot builds up on future releases. Treating it as a necessary piece of a larger whole makes the game far more valuable. So your question can't really be answered. However, if you were to ask me if the 9-nine- series is worth playing and worth the money, then it is would be a resounding 'YES' and worth every cent. My favourite character is a toss-up between Miyako and Sora.