This is the ideal book to use a basis for learning QFT. That isn't to say that it's the only book that you'll need or that it can replace a course on QFT, but the good thing about the book is that it shows you how to actually calculate things. So my way of using this book is to go try to do a calculation until I run into an idea that the book does not explain well. At this point, I turn to some other book for more details. Quite a few times, the other books are not necessarily books on QFT. Ideally you want to be in a short course on QFT that shows you what the ideas in QFT are so that you can go crazy, compute a scattering amplitude and learn the details of the tricks involved in the calculations from a text. Which brings me to the the topic of backup texts to help understand things that this text does not treat well. A great set of supplementary notes for understanding the ideas involved in QFT are David Tong's 'Lectures on Quantum Field Theory' ([...]) that are freely available on the net. Unfortunately, they don't go very far but another great set of notes are Michael Luke's version of Sidney Coleman's 'QFT Lecture Notes' ([...]) that are also freely available on the net. A good text for more basic QFT stuff is Franz Gross' 'Relativistic Quantum Mechanics and Field Theory' (http://www.amazon.com/Relativistic-Quantum-Mechanics-science-paperback/dp/0471353868/). Gross' book assumes less knowledge on the part of the student and spends a lot of time on the EM field and the K-G and Dirac equations. A good supplement for (mostly) classical fields and gauges is 'Geometry, Particles and Fields' by Bjorn Felsager (http://www.amazon.com/Geometry-Particles-Graduate-Contemporary-Physics/dp/0387982671/). At a much more elementary level is Davison Soper's 'Classical Field Theory' (http://www.amazon.com/Classical-Field-Theory-Dover-Physics/dp/0486462609/). Special relativity and electrodynamics are covered well by Asim Barut's 'Electrodynamics and Classical Theory of Fields and Particles' (http://www.amazon.com/Electrodynamics-Classical-Theory-Fields-Particles/dp/0486640388/). A good book for a more laid back, overviewing, historical, pedagogical and well-written view of QFT is Steven Weinberg's 'The Quantum Theory of Fields: Vol I, II & III' (http://www.amazon.com/Quantum-Theory-Fields-Foundations/dp/0521670535/), although, like most texts written by Weinberg, it's great and scholarly for people who already know some QFT but probably not a good text for someone seeing it all for the first time. Finally, mention must be made of the excellent, but sadly out of print, text on QED by Josef Jauch and Fred Rohrlich 'The Theory of Photons and Electrons' (http://www.amazon.com/Theory-Photons-Electrons-Relativistic-Mathematical/dp/3540072950/). Jauch and Rohrlich cover most of QED but none of the developments involving the Weak or Strong Force because they were not understood at all the time of the publication of the first edition of the book (1955). An update in 1976 included more QED but the death of Josef Jauch prevented it from becoming a full-blown QFT text.

In conclusion, you'll probably wannt Peskin and Schroeder as a sort of 'hammer, saw and screwdriver' text (a carpenter's basic tools are hammers, saws and screwdrivers) but you'll need to go grab other tools every now and then.