As much as I like Chaosium, I'd say this 'world' for RPG play comes in at the bottom of what I consider the "Big 3" of Immortal roleplaying (Being Immortal, Unknown Armies, and Nephilim). Plusses - the occult and magic systems are incredibly well thought out, and you can tell hours upon hours of effort was made in making the supernatural beings and powers as based in actual legend / myth / occult 'truth' as possible. This book really does a good job of making the world 'real', and the way the occult and magic fit into the world feels natural in a way that many RPG world-systems dont. Another nice thing about the system is that it's very temporally flexible. It's incredibly easy to adapt this to a medieval setting, or even something along the lines of ancient egypt / assyria / crete, the system can accomodate different times with few problems. Minuses - this is a personal opinion, but the character archtypes and races just did not strike me as very interesting. This might be a consequence of the more 'natural' setting, but it seems to lack the "oooh! I want to play that!" factor that alot of supernatural systems can bring. Character creation, and the systems in general, is also somewhat dry and complex. While Unknown Armies has a very easy system to learn, and Immortal's system is very complex at first but becomes a great asset to play once mastered, the systems in Nephilim seem somewhat dated and 'by-the-book'. Of course, this may not be a downside to you, I actually ended up using this book (and the supplements) more as "idea-boosters" for other campaigns I run in Unknown Armies and Immortal. The world-views are similar enough that ideas from one sourcebook translate quite easily to another series. And while i'm not a big fan of the characters or system, the sourcebooks do contain alot of material to jog the creative mind, which makes them worth a purchase if you are an ST who runs occult / supernatural / immortal themed games.
... and the most realistic, too. I recognize so many things from real occult teachings, it makes the game very fun and engrosing to play. I highly recommend it to those RPGers interested in metaphysical/occult.