It's a fiction that's very well narrated, to the point where I could really lose myself in the story. The descriptions are vivid, the (few) action scenes are short but fast, and the interaction between the protagonists are very credible and three-dimensional.
Seeing that it was written some 70 years before Armstrong and his companions actually set foot on the moon, Wells obviously had no idea whether or not his depiction of our satellite would hold any truth. Nonetheless, his descriptions are all so strongly supported with sound scientific explanations that one might actually believe his version of the moon to be real, perhaps in some other parallel dimension or something. A classic example of the true meaning of Science Fiction, if you ask me.
Beyond that, the story contains some very interesting philosophical issues, such as the disconnection of mind from body that could occur when one floats alone through space in absolute darkness for weeks on end.
It's so rich in detail and atmosphere, so complete and correct in scientific foundations, but at the same time so naive and filled with a childlike wonder that it's really enjoyable on all levels. And to top it off, the ominous, gritty ending is right up my alley.