What I liked: The brief summary of VI3, to set the context of new features in the vSphere release set. The understanding of storage, storage I/O, and protocol access. Table descriptions of the differences in each of the editions and releases. The time spent on vSwitching is helpful. Although I expect to see more on vShield in future vSphere releases release sets & material / books available. There are some areas of VMware that just are not ready to be unfolded as well, but Siebert gives you what's out there today. The time spent here on networking (and again, the use of tables) was helpful. Screen shots and command line references are good.
It is a great book for advanced beginner and the veteran. I've been using and implementing VMware since 2000, and still got a lot out of it. It is geared more towards and administrator or an implementation specialist, which is what I would expect. Anyone on the design or consultant team will gain value as well, since it helps you dig deep enough to be successful (plus there are practical sections on install/upgrade options and methods).
Anyone needing advanced deep dive in specific subject matters / topics covered will at least get some direction on where to find it, in the "additional resources" areas before each chapter's summary. For example, I am a storage expert -and would have like to dig deeper in the IOPs, calculations, boot storms, and media types, etc -however that should be in an entire sub-category / specialist book . For what is needed here in this book - things are quite sufficient and all categories of design are covered very well. If you want those deeper level resources... you can likely still find them from the author, since he is available on line (twitter, etc) too.
Last note. If you went to VMworld, OR if you wish you did, and heard about all the next generation VMware release sets, then you still need to get your base in vSphere 4. This book covers those bases so you can be successful now, and adequately prepare for that next generation. If you are new to VMware and about to go to formal training, certification, etc -it's best to prepare by reading this first. You want to understand this topical coverage now & at your own pace, and hear it from a very practical perspective, before you hear it for the first time somewhere else. You will also be using this book later as a reference. That's how easy it is to read & use.