After the underwhelming "The Premature Burial", "X", the second Roger Corman-Ray Milland collaboration, really delivers, largely because Milland is better cast. In fact, in his autobiography he rated it as one of only two worthwhile films he made out of nearly 100 in a long career, the other being "The Long Weekend".
As Corman points out in an exceptionally good "extra" director's commentary, after the war obsessive scientists were always seen as Bad, and it was only after Man went into space that the idea of the Good Scientist became current again. Dr. James Xavier wants X-ray vision purely to be able to cure people, but as he gets more involved in taking the drug which gives him the power, it becomes literally blinding. On the run for accidentally killing a collaborator, he is reduced to being part of a freakshow, and ends up after a hair-raising chase in an evangelical tent which provides the disturbing, almost apocalyptic ending.
The special effects are almost laughably cheap, mainly created by irises and dissolves, with some basic film processing thrown in. However, it doesn't matter at all, because we have swallowed the McGuffin about how this vision is possible. And that's what the audience wants to see - X-ray vision. We buy it wholesale.
This is on one level a simple science fiction story following the Dr Frankenstein tradition, except the monster is himself. But it is also a film about drugs (LSD was gradually coming onto the market in 1963, and Corman originally wanted to do an explicit movie about that). And in a curious way it becomes a religious movie too, about the possibility of either becoming or seeing God.