It's not the easiest thing to describe (as it seems to be with all things Neptunian) but I'll give it a shot.
It felt like my mind was just becoming aware of all sorts of new ideas about life. I went from having a clear cut image in my own mind of what was possible and what was not, to accepting the fact that anything is possible. This reversal in beliefs came rather suddenly, and along with it came a vulnerability to believe in things that were irrational, illogical, and downright false. These illusions that I held to be true were so strongly rooted in my head as real that no thing, act, or person could make me believe otherwise.
Some specific examples: My girlfriend at the time was cheating on me, had been cheating on me, and EVERYBODY knew about it. Everybody was telling me the truth about what was really going on, but I did not believe one lick of it. I wasn't even really in denial, it was more of a subconcious self-deception.
I went through different periods where I thought certain friends of mine were turning on me, to the point where I was actually frightened of them. But they had NO ill will toward me at all.
There was more than just interpersonal relationship illusions, but I'd rather not talk about those.
Throughout the entire transit I went through periods of awe and elation, followed by confusion and despair. There were such intense surges of gain and loss; like nothing I'd experienced before. But, I'm an entirely different person now- for the better.
My advice to you, because this is the advice that the interpretation on Astrodienst gave me, is to study metaphysical, occult, astrology, or depth psychology related subjects to get a better grasp on the lessons you'll be experiencing.
I hope I didn't make it sound like a horrible time, because really it was by far one of the most fun and interesting periods of my life. Just keep your head on straight and you'll cruise right on through it.
