I've studied Quantum Field Theory from a variety of texts, notably Srednicki, Peskin&Schroeder and Weinberg I & II (and my lecture notes from university). I firmly believe that QFT in a Nutshell is not suitable for learning (most of) the techniques and methods that were crucial in the development of QFT and its experimental applications.
There is, however, no other book that enables one to look back at what one has studied with a wide-angle lens to appreciate the foundations and consequences of these developments. I would personally recommend QFT in a Nutshell as a "review guide", encompassing all of the most important results and aspects in a simplified (but nonetheless complete) form. After a long study session I would often find myself reading the relevant section in 'Nutshell' to see what Professor Zee has to say on one topic or another. In addition to reviewing the material, more often than not I would find a deeply enlightening argument written in the author's trademark colloquial style.
Also, the book excels at introducing modern concepts relevant to students of QFT (such as its applications in solid state physics or the modern numerical techniques presented in section N of the book) at just the right level for future investigation through primary source material.
The only complaint I have about the book is that an insufficient number of references to primary sources is given for the main content of the book, in case an interested reader wishes to delve further into a particular technical problem or a phenomenological consequence. However, there are references in the footnotes every few pages about particularly difficult or interesting topics.