I was just saying on another forum the grimoires do us a bit of a disservice by presenting the demons as a D&D 'Monster Manual' populated by distinct personalities and beings. While in ritual it is useful to approach and treat them as fully distinct individuals, but they are truly Legion.
What the grimoires mostly do, in my opinion, is provide stories, personalities , and myths that structure the type of manifestation and experiences you'll have with the daimons. (This does not mean that the myths are "untrue" btw.) Modern simplified Goetia... ala GOM, S. Connolly et all, I hear can work, so all good. The GV too can provide even newcomers obvious and fast results with a few of the trivial operations. The full GV working is not required for results magic, but then again, neither are demons, so the question becomes what you'd like to experience over the course of your magical career.
The ritual framework you choose structures to a certain degree the effects and types of manifestations you'll have. The Grimorium Verum, as well as other "pact" - based grims are deeply chthonic in form, and create a ritual context that is similarly chthonic and outright paranormal. The most dramatic results, including theophanic manifestations, - and useful ones, not just poltergeist thrashing about - have been in using GV's protocols. Conversely the Lemegeton , which I admit to not having much experince in, I find more "stellar" and mental in effect. That said, as always, YMMV.