It's kind of difficult to know what to say about this book. It's a complete overhaul of sociological thought from the very basics up. This makes it a very dense and demanding read which took me about 6 months. As Luhmann says in the introduction, the book is intended as a "paradigm shift" for sociology based on a similar paradigm shift in general systems theory and second-order cybernetics. The level of theoretical complexity is uncompromising and unrelenting.
It's impossible to summarize it's main "argument" here without without bastardizing it. But in a nutshell, Luhmann suggests that society should be studied as a plurality of operationally closed systems, each with their own specific functions, codes and system rationality. The book is therefore an explanation of how social systems autopoietically reproduce themselves; how they generate their own structures, how they differentiate themselves from their environment, how the non-triviality of social systems means they will always feedback into themselves in unpredictable and uncontrollable ways, and the consequences of this. It offers rigorous reformulations of important concepts like "meaning", "action", "communication", "interaction", "causality", "time" and more. It culminates in a general statement about epistemology which both relativizes the whole book, while at the same time calling for ambitious social theoretical investigations into specific social systems, as well as the system of society in general.
Is it successful? The sophistication of this book makes it very difficult for me to judge. I've never read anyone else in sociology who is anywhere near as penetrating a thinker as Niklas Luhmann. He tries to leave no stone unturned, even while incorporating the impossibility of doing that into the theory itself.
Thinking alongside Luhmann has been enlightening. I've been forced to completely re-examine very basic sociological concepts or ideas which I had thought I understood very well, but I now realize I was using in vague or tendentious ways, ways which really only served to disguise the uncertainties or inconsistencies of much sociological thought.
The effect of Luhmann's work on my thinking has made me both more critical and skeptical of many sociological claims, while at the same time making me more open and accommodating to the multifarious work that sociologists do.
I haven't fully made up my mind whether his theory of social systems is the "way forward" for sociology. I partially agree with Jurgen Habermas that Luhmann's work objectifies society in ways which tend to obscure or diminish the participant's perspective. But I also appreciate the rigor and depth of analysis that doing this allows.
For anyone interesting in starting out with Luhmann, I would not recommend beginning with this book. I would recommend the volume titled "Introduction to Systems Theory", which is a series of lectures he gave in the early 1990s. That's a book which covers similar ground, but is much more simplified and accessible by comparison. It should give the reader a good sense of whether it is worth the significant commitment of intellectual energy and time it takes to tackle "Social Systems".