I agree it depends on what you need to improve.
I liked the number properties, the geometry and the SC ones. The RC and CR ones aren't that useful, if you have a good grasp of English. I think doing problems and reading more will increase your abilities more than the guides. SC is really good bc, if you're a native English speaker like myself, you probably get used to everyday speech which is full of grammatical errors. The SC guide does a good job of breaking down all the rules tested on the GMAT. For math, personally, I needed a refresher in geometry. All those forumlas and rules that I learned in high school, I've forgotten. I think that's the case with most people, so the Geometry one is helpful for that.