I definately think it was a mystical experience of some kind.  Like others, I do not think that it means (or even that Thomas meant) that his writing is/was worthless.  He was just comparing its value to that of his "revelation." 

Reminds me of Job, who argues through many chapters that his understanding of God and of his own innocence was correct.  We are told that God vindicated Job's understanding and condemned that of his friends (Job 42:7-8).  Yes despite this Job's final response is, "I retract all I have said and repent in dust and ashes". (Job:42:6).

Seems for some, such experience is so overwhelming that all else pales beside it.