I spent four days in Seoul with this guidebook, and I was mostly disappointed with it.
On the positive side, there are some fairly good restaurant recommendations - Gorilla in the Kitchen turned out to be as good as described, and a bibimbat joint in Itaewon was good, although the directions to it weren't terrifically helpful.
On the negative side, it's very hard to use. The book is grouped by theme rather than geography, so if you're trying to find out what to see, where to shop, where to eat and what bars to visit in one area, you have to keep flicking back and forwards through the book. Consequentially, the maps aren't right next to the descriptions of the things they see, so the legend refers to a set of places that then require further to'ing and fro'ing to figure out if they're interesting or not. Strangely, Time Out guides use pretty much the same approach, but don't produce the perception of having to flick back and forth all the time.
Ah, and that brings us to the maps. They're terrible. Hard to read, with no Korean script on them (only in the key), with most obvious landmarks omitted and locations of the points they do mark not clearly on one street or another. The map of the subway system is on a folding pull-out map, which would be a nice idea (since you wouldn't have to carry the whole book around, just a map) were it not for the pull-out map having hardly any features on it whatsoever, and displaying just the centre of Seoul on such a large scale that it's useless for navigation.
The quality of the writing is so-so. Perhaps to compensate for this, Lonely Planet keep most of the guide printed in black and white and grey on dull looking paper.