Sometimes i recognize that a story went "of the planned path", especially if it causes inconsistencies with earlier chapters, or a character changes its personality/way of acting very much. That only distracts me if it is really out of line for the character.
As for "planned" or "unplanned" plots, its hard to tell for me as i am not a writer, but i suppose you need to have a general plan, a framework for your story. Not too tight, so your characters have space to grow and change (which is only natural, gives it a human touch). I don't mind at all if a story takes unexpected turns or end different from what i expected. If at the end it still all fits together, i like to be "lead around by the nose" (if that is a proper English term ).
Hmm, did this make any sense ?