After finding my students struggling with style, I began searching for a style agnostic guide. After reading this guide, I decided to add it as an optional reference for my undergraduate students. I have found that using a supplemental quick reference helps students better understand issues and the writing requirements.
This is probably the perfect guide for undergraduate students. It is well written and a good general reference. The manual covers APA and MLA, but at a fairly high level. With that being said, I believe it is specific enough for students to get through a variety of papers in various programs. However, it may be necessary to still supplement with other style guides or web based references. This is a great general reference that addresses various styles, but is better for general grammatical issues.
I would not recommend this for most graduate students though. While we all need a little reference, graduate students should already have a mastery of most of the concepts within this text. When addressing the specific styles, I would suggest using a style specific guide or referring directly to the MLA or APA guides.