This is the first book in a long time where I look at it and regret buying it at full price, but only because of how sparce it is in size. Part of me is kicking myself for going full retail, but the other voice is saying "what's done is done." Embracing the second voice, I dig this book. I have no plans of running it in the Realms (fortunately I have a large cosmopolitan jonce that I've been itching to delve under) but it's still useful.
Best part: General edition neutrality. While magic items specifically have detailed 4e stats, most monsters are not statted out, but referenced by book. When DDN gets released, I can see myself going back to this book (and previous Undermountain stuff) and use it well. I assume this was a deliberate choice informed by the looming DDN push.