You could argue the plot is a little 'B movie' with a strange radioactive sea-fog incident leading to a bad case of the shrinks for our hero Grant Williams, but it's done with a passion and attention to detail that means you rarely question the realism of the situation. The special effects are really impressive considering the fledgling nature of techniques at the time and the scenes in the basement are especially gripping and realistic although the cheese does have an uncanny resemblance to a large block of polystyrene.
Williams portrays the emotion and turmoil of the situation perfectly; contemplating suicide, seeking a relationship with a woman of similar height and ultimately culminating in the uber-philosophical 'I still exist' ending.
Apparently a Hollywood remake is in the pipeline, which fills me with a sense of terror at the prospect of CGI spiders in an all action, fast paced romp, similar in style to another Matheson adaptation 'I am Legend.' Ignore the fact that this film is in black and white, uses early special effects and a questionable plot, and enjoy the literal descent into another world.
If you like this you could also try:
The Thing From Another World, Day of the Triffids, The Day the Earth Caught Fire, The Omega Man, Village of the Damned.