I watched the film when I was younger and remembered enjoying it a lot. At a later point after I discovered cannabis I decided it would be a really good idea to get high and watch it, hoping it would blow my mind and bring on a whole new level of meaning. I found my experience was closer to yours though, the whole film suddenly felt really pretencious and shallow.
My recommendation is to just watch the film without having too many preconceptions. Just let it evoke whatever it's gonna evoke, it's still a whole lot of fun and the animation and music is great. I really like the soundtrack. (Which should be available for download if you google 'a scanner darkly soundtrack mediafire' assuming you already own the soundtrack and would like a digital copy)
Thinking back there are a lot of memorable and fun moments. Good conversations from good people. Just don't take anything too seriously.
