Out of curiousity, when considering my last trip to Korea, I purchased this book, Moon Handbooks South Korea. I already had the 6th and 7th editions of the Lonely Planet Korea books, and am a self-avowed fan of Lonely Planet. On reviewing the Moon book, and comparing it to the LP products, I have to admit to being impressed by the breadth of coverage of Korea by the Moon Handbook. I lived there for a year (in Gumi, near Daegu), and have visited twice more for several weeks, but there were places mentioned in the Moon book that I was totally unaware of, despite spending several days in some areas with my LP guide book. In fact, Gumi isn't even mentioned in the LP at all (except on a map), while the Moon book does have a brief section about the city, which surprised me, as the only thing of note in Gumi is the fact that it is the birthplace of President Park Chung-hee.
While I would consider the LP books are slightly superior in some areas, such as "Facts for the visitor", "Getting there and away", and "Getting around" sections, for sights to see, the Moon book provides much greater depth and information. This is particularly the case for cities and regions outside of Seoul. Of course, this is partially due to the greater number of pages in the Moon Book which obviously allow greater amounts of information to be provided, but having said that, in size and weight, the Moon book isn't significantly larger than the LP books. Otherwise, in terms of content, Moon is similar to LP. There is a Korean language section at the back, and place names are given in hangul as well as english in the text. In some cases, information as to how to get to some places is relatively light, but this probably wouldn't be such a problem to a long term visitor.
For short-term visitors, the LP book will suffice as plenty of information is provided that will keep people with a limited amount of time in the country busy. But I would suggest for those planning a long term stay in Korea (for example as ESL teachers) to try out this book due to the higher amounts of sights listed. Not every teacher gets to work in Seoul, and one of the shortcomings of the LP book is its brevity of information provided for the smaller, but still significant cities of South Korea. This is where the Moon book really comes into its own in comparison to the LP book, as it does not stint in this area.
To detract from the book, the maps in some instances weren't as well or exactly detailed as the maps in the LP book. The book is also directed at the US market, so in some areas of the book, such as "practicalities", information relating to visas, airlines, and exchange rates is relatively light for readers from the UK, Australia and New Zealand. Still, I would recommend this book over the LP book, at least until the next edition of the LP book comes out, which I believe will be around May 2010.