Sometimes I only read the comments, because the headline makes it sound like people's reactions will give me more information than the article itself: an example would be "Library X was updated". But in any case I will never comment without reading the article first, which I think is not an ahem, universal practice among all HN readers.
Most programming/engineering content I'll read the comments first to get a sense of the level/quality of the post.
For math, science/physics, tech inventions I'll read the post first, unless I'm on my phone then the comments load and read faster on my little screen.