This massive book on QFT is a standard text nowadays and used at many universities. The book is extensive and very detailed. If you manage to follow the text and keep up with all the nitty-gritty details, then you are well underway into mastering QFT -- but this is quite a challenge. The book is great for QFT when applied to high energy physics, but less so from a condensed matter perspective. The chapters on renormalization, symmetry breaking and gauge theories are very thorough.
It can be quite difficult to keep a bigger picture of what you are exactly doing (and why) at any given point throughout the book, as you can get easily lost in the sea of formulas and details of the calculations.
The book is lacking in some topics. For instance, the treatment of the functional integral formalism is somewhat poor. You also need to use other resources for the representation theory of groups (Lie groups and the Lorentz group in particular), because it's not really treated well here.
Still, it's already a classic and a must-read for any QFT-enthusiast.