This movie really grows on you. Yes, it's true, the non-3D parts are boring, but I find them functional. Like any good freak-out movie, if it was all a freak out, you'd get worn out fast. This is like a freak-out musical in a way, where the freaky scenes are the equal of big musical numbers.
A doctor is turned on to a creepy mask by one of his patients who has turned into a homicidal maniac. Next thing you know, the doctor is trying on the mask and going insane, and oh what a mess, and his girlfriend and everyone, and blah blah blah.
Meanwhile, in the hallucinogenic mask sequences, you get to experience what it must have been like to be on LSD in the 60's. There's a whole "alternate world," where a strange, mutated man is milling around, looking for the woman (?) he is in love with in a fever dream landscape where there are skulls and burning hands and satanists and gore and other neato stuff. And it's all in bizarro 3-D! Even if it doesn't work well all the time, it's still mighty disturbing, especially for a movie from 1961! The images, and the incredible, collage-like soundtrack to the freak sequences will linger on your brain long afterwards, in the same way that wearing those horrible glasses leave an impression on your eyes after you take them off for the "normal" scenes. You're exhausted, and confused, and weirded out.
Yay