Let's get one thing straight, every single C# developer with a working knowledge of the language should read this book. I picked it up because it had been three years since I'd written any C# (I'd been doing Java) and back then all I had exposure to was version 1.1. I knew nothing of var, LINQ, Lambdas, extension methods or dynamic typing. I guessed a little about generics from my Java knowledge. I've read the book and now I know about all of them. Of course I don't know everything about all of them but I know enough to make use of them. If I went back and read the book again and again I would increase my knowledge of these features with each read.
This is not an introductory text. The book starts off with a a brief tour of C# 1.1. It then goes through all of the recent versions, 2.0, 3.0, 3.5 and 4 introducing most of the new features as it goes. It does what it says on the cover, it goes into each feature in depth. This is both the books strength and its weakness.
There is so much information in here that at times it is very heavy going. I will admit that I skipped a few sections when I felt that I knew enough and knew I could go back to it at any time. If I'm brutally honest, I feel that this book could be pruned quite a bit and still be in depth. Alternatively, maybe a text that gave an overview of the various features with some examples would have been more suitable for me.
Jon Skeet is clearly a C# junkie and very passionate about the language. Jon, you'll be glad to hear it does all make sense!