This book isn't what I think of when I hear the word encyclopedia. Rather than short superficial articles, it's a collection of 47 detailed chapters covering everything I would imagine most people would want to know about the solar system. I only read two-thirds of the book, but I thoroughly enjoyed the parts I read.
This is a high level view of the parts of the book that I read. The book opens with three chapters covering the solar system as a whole. This is followed by chapters covering the Sun, each of the planets (some planets with multiple chapters), Pluto, an overview of the planetary satellites, chapters for several of the bigger satellites (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton), meteorites, asteroids, comets and the Kuiper belt.
Although the chapters were written by different authors, the quality was uniformly high. The text was well written and as far as I can tell very complete. There were also many informative diagrams and photos. Many of the photos were quite stunning. One example is the photos of a patch of Europa's ridged plains, the details were incredible. Another nice feature is that the book justifies how we know what we know, for instance it explains the reasons it's believed that some planetary satellites have liquid water under the surface.
This book is also fairly up-to-date. One of the main requirements I had for buying this book was that it included the information gather by Huygens on Titan. I wouldn't have minded a few more pictures from Titan's surface. Obviously in a dynamic field like this no book is completely up-to-date.
I didn't think there were any significant problems with the book. One word of caution is that there may be more details here than some potential readers might want. To fully understand all the material in the book one would have to understand basic Newtonian mechanics, some electromagnetism and a little bit of chemistry. Even without this background one could still understand the vast majority of the book. Although the chapters were written by different authors the amount of repetition is pretty small.
Just to be complete I'll outline the contents of the third of the book that I didn't read. Although these parts looked good from a quick glancing through, I didn't read them in detail (I do hope to have the time to do this later) and can't comment on them. They are solar system dust, four chapters on viewing the solar system in the non-visible spectrum (x-ray, ultraviolet, infrared and radio), ground based telescopes, planetary radar, remote chemical sensing, regular and chaotic motion in the solar system, impacts, volcanism, astrobiology, exploration missions and extrasolar planets.
In summary, I think this is an excellent book covering a huge amount of material, often in great detail, on the solar system. I would expect it covers just about everything potential readers would want to know.