Among the disadvantages are the tendency to fade away into faerie; among the advantages are the ability to make use of faerie powers that are hard or impossible to duplicate using Hermetic magic.
Some spells require a greater degree of Feyness than others. If one character knows a spell (or is attempting to spont it) but lacks sufficient Feyness for it, an entity with greater Feyness may effectively lend its Feyness in a process somewhat like lending Faerie Sight with a hand on the shoulder. The process is not well-defined, but requires cooperation between the two entities. (It can be a hand on the shoulder, verbal coaching, and similar such stunts.) The amount of Feyness lent varies in the amount of interaction and the quality of powers wielded by the faerie. (A high-Feyness Merinita acting as lab assistant can provide an effective season's worth of coaching on the spell-- but only for themself as coach!)
In the Amurgsval campaign, we have a system for tracking how fey a character has become. It is numeric, much like Twilight, and the scale may be comparable.
|Fey personality trait (based on Faerie Magic)||+score|
|Faerie Cord (to familiar)||+(cord score - 1)|
Faerie Magic is known to give you a Fey personality trait; this is not required, however, in order to acquire said trait. This causes Merinita magi to walk a knife's edge: too little Feyness limits access to faerie power, while too much may cause them to vanish into Arcadia.
Binding a faerie familiar requires establishing a Faerie Cord at a score of at least +1; this costs 5 points, which is the minimum Faerie Magic score for binding a faerie familiar. +2 or +3 increases a character's Feyness by +1 or +2 respectively. (A +3 gold cord to a faerie familiar [as per the Wizard's Grimoire rules] increases Feyness by one category for purposes of spellcasting without increasing it for other purposes.)
Here's a rough chart of what sorts of power require what level of feyness. Spells are listed in italics, faerie powers (from Faeries 4th Ed pp130-132) are in bold.
|1||Most spells easily replicable with Hermetic magic at no more than a rank higher; Eyes of the Cat, Gaze of the Thousand Pinches,Aquatic, Change, Fascination, Greater Flight, Make Animals Tame/Wild (temporary)|
|3||Peculiar spell durations (Journey), faerie-status related powers (regio entry, ownership); The Billow of Eiderdown, Mark of Faerie Protection, Awaken the Mirror's Self, Mask of the Beast, The Faeries' Touch, Feast of Delights, Greater Glamour, Improved Characteristic, Invisibility, Speak with Animals, Very Tough|
|6||Healing power, bad luck, moderately powerful shapeshifting, Faerie Sight-like activities; Breath of Invigoration, Fortuna's Frowning Gaze, The Avian Form, Hex/Blessing, Healing (using vis in some way), Shapechange|
|9||Awakening power, strong transformations; Awake from the Sleep of Ages, Tongues of Ancient Ages, Banishing the Faerie Curse used on Faerie magic, Ennobling the Base Metal, The Weakling's Revenge, Summoning (using vis), Lord of the Wild Forest|
|12||Danger sense; Banishing the Faerie Curse used on non-Faerie magic, organ transplants, Equalize Resistance|
|15||(Typical church faerie)|
|18||(Typical Sidhe, very few player characters) Summoning (without using vis), Make Animals Tame/Wild (permanent), Healing (using something besides vis, such as fatigue or someone else's wound levels)|
|21||(Tuatha de Danann) Resurrection|
|24||(One of the original elemental powers of Arcadia)|