You added a second topic that clouds the whole discussion.

Everyone will agree that sitting in typical airplane coach class between two huge others is certainly a miserable experience but from this reply one can no longer draw the conclusion that sitting in a middle seat in itself is a miserable experience.

Specifically regarding someone occupying your seat, that topic is worth complaining about on the spot (prior to departure) and there is a chance that, in order to get the flight out sooner, the crew might end up solving the middle seat topic for you in addition.