Like the previous reviewer, I couldn't imagine why I would want Low-Tech. Wasn't the material provided in the Basic Set, Fantasy, and Banestorm sufficient? I'd run games for years in low tech settings without needing the book, so why did I need it now?
Now, after finally getting Low-Tech, I can't imagine running a medieval game without it. It turns out the medieval weapons in the Basic Set are, well, really basic. There's tons of interesting weapons and gizmos that you never knew existed with sufficient background to give the player and GM context about when they'd be appropriate.
What is Banded armor and who used it and why? What is the effective difference between a Katana and a Broadsword? What were the early types of gunpowder weapons like? All of these questions and more will be quickly answered if you buy this book.
But wait, there's more!!
This book would have been double its current size if Steve Jackson Games hadn't made a marketing decision to keep this book skinny. Instead they put their very useful and very detailed information about Kingdoms and Philosophers, Weapons and Fighting, and Common Life in three separate PDF available for download on their website. I've bought the first and the third supplements and can attest that they are REALLY good.
Now go out and buy this book and GURPS Horror and make Evil Stevie and his merry bunch of game-writers happy so they'll write more good stuff!