Overall pretty good. I have had good results with rituals 1 and 2, but never really bothered with 3 due to the overhead required for that. Ritual 1 is a bit overkill to try and do with all of that letter-to-letter transcription, but it does work. I find ritual 2 to be the simplest and quickest to do overall.
The results with this book have been substantial and the technologies inside of it, especially the evocation keys + words of power, are potent in my eyes. Just using these parts in my own customised rituals lead to some good effects for me for feeling the spirit. In terms of results, I managed to do a lot of good stuff with this book, though I admit I customised my approach with offerings and also added some sympathetic magic elements into it where I deemed it necessary to help focus the ritual better. But overall, it is a great addition to any practicing Goetic occultists library. I feel that "Gordon Winterfield" provides a lot of good information on how to view the Goetic spirits, how to work with them, and perspectives on how the magick can manifest results. There is a ton of information in there that can also be used in your own custom rituals in order to beef them up to if you wish to stay in your own practice.
The most powerful effects were always with my own custom rituals adapted from the original grimoires, using the technologies from this book, the old solomonic stuff, and GD stuff. But these rituals always took 2-3 hours to do and are not very time efficient. Demons of Magick is a good balance between quality of results and time efficiency. So yes overall, an excellent book for working with the goetic spirits.