Allen takes a collection of very simple concepts and puts them together to form a larger system which he calls "GTD". The GTD system in it's entirity does feel a little complicated at first read but I would definitely say this is one of those books which needs to be read over a few times to understand it properly. In the first part of the book he explains the system in brief and goes on into more detail about each concept in the later chapters. This makes it feel like there is a bit of repetition at times but I think he is just trying to reinforce the message. There are lots of great quotes and inspirational messages dotted around the book to help get you into the right mindset.
I don't think this system is for everyone. You can dip in and out and learn some 'tricks' as he calls it but to really get the best out of the system you would need to be very disciplined and committed. Also, I think level of benefit would depends on the job you do or how busy your life is.
On the one hand, this is a 250 page book about making lists. On the other, a clever system system which could change your life (well, in the getting organised sense anyway). His methods have become widely used and he seems to be well respected in the field of time management / consultancy (check him out further on you tube) so he must be doing something right.