The best way to get a summoned demon to do one's bidding is to bind it. Binding is a dangerous proposition for any would-be sorcerer and it cannot be taken lightly in the game. This makes for a dramatic build up to the binding roll and a great deal of forethought by the player to summon exactly the right type of demon.
Almost by necessity, these factors bring about a more narrative approach to using magic than most games I've played. A good example from last weekend's Kato, a Demon of Knowledge summoned and bound by one player's character, Prefect Thule. Thule had a 95% chance of summoning such a demon. The summoning is a straightforward roll of d100, though I allow up to +/- 10 for circumstances and special preparation. So Thule had a very easy time summing the demon. Next, however, the player had to define who and what is this demon so it can be bound and put to use.
Thule's player decided he wanted a demon who heard and understood court gossip from a multitude of planes and kingdoms. So his character cast his gaze through the Hells until he found Kato, the demonic curator of Duke Arioch's morbid museum. Kato is refined and dignified; but, a terrible gossip. His vast intellect stretches across 47 prime planes and through 9 Hells. He hears all that any courtier whispers on each of these levels of existence. Through such keen hearing he is able to learn, or guess at, most anything that transpires in courts great and small. Kato takes the form of a thespian mask; but, as Chaos is his very nature, the details of the mask are constantly changing to reflect the focus of whatever he might be overhearing at the time. When asking Kato a question, Thule dons the mask and then, should Kato know the answer, hears a distant whispering answer to his question.
Mechanically, the character just summoned this demon of knowledge and determined its statistics. The sum of all the demon's statistics had to exactly equal the sum of all the summoner's statistics, with some restrictions thrown in around minimum levels of magical power. The demon is then bound to an object, in this case the thespian mask.
Demons of Knowledge have a good chance of knowing the correct answer to just about any question asked of it. After all is said and done, Kato had a straight 47% chance of knowing any answer. Its a simple as that. However, because of the color added to Kato by Thule's player, all of Kato's answers were couched as gossip and gleanings overheard at the various noble courts and posh bathhouses throughout the multiverse. It made each question and answer, while mechanically a very simple d100 throw, a colorful and imaginative narrative.
So I'm quite pleased with this Stormbringer magic system. It forces a creative narrative just to be able to effect magic, which pushes the game in a very different direction than if the system were more like the Vancian magic of Dungeons & Dragons or the point-and-level-based system of Rolemaster.