TLDR AT END
The setting of Ghostrunner is a dystopian cyberpunk city. Gameplay-wise, it's based on trial and error. Think Hotline Miami, or Katana Zero, but in first person.
The level design is fun. There are usually multiple routes to take, and they move FAST. Varied enemies keep it interesting, but annoying at times. There are a few badly designed levels in my opinion, being the latter two boss fights and the final level, but they are mostly intuitive and fun to play.
Some levels have combat-free puzzle sections at the end, which I wished were skippable. They aren't enormously difficult to solve, but I still recommend looking up the answers so you can get back to the bloodshed as quickly as possible. Difficulty on the whole is sort of a non-issue. The game was challenging to me personally, as it was my first game on keyboard and mouse, but I was able to complete it in a reasonable time.
There isn't much story to speak of, and that's okay. The gameplay speaks for itself, and being about high-octane combat as a cyborg ninja is cool enough for me.
TLDR: Exciting, challenging first person combat in the style of Hotline Miami, with some boring sections in between. Pretty graphics, cool music, bare bones story.