Before I talk about my thoughts on 1.5, I need to clarify that I am a huge fan of Ghost in the Shell, so I may have put a bit too much of my thoughts into some categories. Ghost in the Shell 1.5: Human-Error Processor is the 3rd part of the GITS trilogy, so if you haven't read the first two [I'll say why reading GITS 2 is worth it in a little], I'd recommend doing so before reading this one. The manga should be read in this order: Ghost in the Shell -> Ghost in the shell 2: Man-Machine Interface -> Ghost in the Shell1.5: Human-Error Processor

Story: I think that GITS 1.5 is necessary to understand GITS 2. The reason why is because it gives you [to some extent] a few little bits of info that add on to parts of the more intricate bits of volume 2. If you didn't enjoy GITS 2 as much as you thought you would, I'd say that it's justified by reading this.

Art: As in Shirow's other two GITS installations, the art-style is more or less the same, and if anything, they've gotten more detailed. My copy is the Deluxe Edition, so I have it in Right-to-Left format, with Japanese sound effects instead of English ones. [Personally, I prefer this.]

Character: There are some new characters that appear in GITS 1.5, although I'm not going to spoil who they are. The already existing characters look pretty much the same, but there are also some new gadgets we see being used.

Enjoyment: I loved reading 1.5 because when I was reading GITS 2, I was confused and had issues with keeping up with what was happening. 1.5 goes back to how GITS 1 was, and was much easier to enjoy and follow along.
I thought some of the scenes were funny, but what I enjoyed the most were the little side notes written by Shirow, which talk about things like how cyborgs interact with certain things. He also went into detail about events mentioned in conversation that readers didn't know much about.
My favorite part about that was the little side note translations for street/shop signs, and for the smaller, not-in-speech-bubble comments made by characters.

Overall, I recommend this 100%, but please read the other volumes before this one.