Talk about a book with a hidden side. Reading the introduction, it hit me that the author was describing a path that, frankly, has an esoteric/magical side. The language is all there and persists throughout the text. Looking it up, I see that Ramha"l is absolutely documented to have been involved in the Italian magic/alchemy scene. Anyone without a background in that literature can read Mesilath Yesharim as an ethical how-to book, but if you do have the background, you'll see that there's a whole other conversation happening beneath the surface.
As an ethical manual, it is beautiful, but Jeffrey Cohan's review said it best: this is not well-suited to individuals prone to mental illness. Additionally, it is very difficult for us modern Jews to really envision the kind of life Ramha"l is prescribing because it is so easy to conflate a simple understanding of his stated goals with ultra-Orthodoxy. That path that, on closer inspection, couldn't be more opposed to the spirit of honest, ceaseless self-inquiry. For additional proof we can look to Ramha"l's own modelling of his ideal path in his own life, which was as a beardless, banished rabbi/playwright who communicated with spirits and delved deeper into the identity of the Divine than any of his contemporaries could stand. So readers, beware of anachronistic association!
A final warning: if you have even the faintest scrap of Hebrew, do not read this book in English alone. Get at least a bilingual edition. Ramha"l's Hebrew is shockingly modern (the Vilna Gaon praised its clarity) and you'll be surprised at what you can get through... and what damage loaded, Christianised English terminology can do to the original intent of this work.