Although I am not a motorcycle rider and I have never used psychedelic drugs, I still found this to be a great movie. If you have allowed these two elements of the film to keep you from watching it, I highly recommend putting those thoughts aside and viewing this.
The main idea of the film is freedom. Peter Fonda and Dennis Hopper take a motorcyle trip across the USA while heading for Mardi Gras. Although this is the destination, Peter Fonda (is looking for something. This something could be America, but you feel it is more profound than that. You receive hints from the things he says to others.
For instance, he and Hopper have stopped at a farm to fix one of the bikes and to eat. After talking with the farmer, Captain USA comments about how great it is to be here on the land doing your own thing.
Jack Nicholson has the best lines of the movie when he explains that we love to talk about freedom, but we are scared to death of people who actually live it. A very important idea considering the war that was going on at the time of this movie.
Again, I highly recommend watching this movie.