I read the version translated by Juan Mascaro and I want to talk about his treatment before moving onto the work. I found Mascaro's introduction to be a bit flaky. Mascaro is fundamentally a mystic who tries to see all religions and most great figures in literature and philosophy as reflecting the same one mystical reality. While of course there is some merit in that view, it hides more than it reveals. In the translation, this gave me some concerns where Mascaro is using Christian language and I was not sure if it was accurate translation or a reflection of Mascaro's philosophical agenda. Also, apparently the entirety of the Upanishads is about equal in size to the Bible and its hard to tell then how reflective this sample is of the whole. Having said all that, Mascaro's translation is good and has a lot of literary power in its plain language use. It seemed to me a well worth reading selection, but I was left with the intent to read the whole of the Upanishad's at some point in the future.
The Upanishad's themselves are good. The selections in this book are very much the core religious doctrines, but their expression is beautiful and they were a pleasure to read. In many ways I preferred them to the Bhagavad Gita which is really my only relevant point of comparison. I do think that they are more interesting and insightful than most other religious texts, although I am at heart a Spinozist and I was often struck by how close they were to the doctrines of Spinoza. Probably every person who was to be literate and understand the major ideas of the world religions should read them. They are also very pretty. However, the Bible could be made to look a lot more pretty than it really is, if a selection this small was made of it.
Anyway, you should probably read this book, and I should probably read the whole thing at some point in the future.