I'd like to hear from folks who have played or SGed magi with the noted magical leanings.
After reading over Theurgy, it feels somehow... off... to me. The mechanics seem to fly in the face of dealing with spirits as Ars Magica has established, and the different is jarring to me.
On the other hand, there's Spirit Magic, which seems interesting to an extent, as long as you're ok seeing spirits as resources to be used. Odd, then, that many Mystery Cults seem to have the Spirit Familiar initiation featured after Spirit Magic.
Regardless, how have people found these two aspects of the Mysteries in play - both mechanically, and in terms of stories generated?
I note a Theurge in the latest Sub Rosa... Someone must have some experience with them.
How so? I'm asking because I'm curious, not because I aggresively disagree.
Theurgy is high ceremonial magic - in many ways much closer to historical "hermetic magic" than what we normally use.
It focuses on names and ceremony - and on gaining access to power by invoking those who have more than you.
Admittedly, some of the entities it invokes might be thought of as faeries (in that they are pagan gods), but these are not the main "targets" for you invokations. Instead you'd invoke the myriad of other "higher spirits", eg. the titans and their ilk.
A much less upper class tradition of magic, spirit magic mainly deals with spirits weak enough to be bound to the caster's will. But even these entities are (sometimes) sentient and may earn the friendship of a magus. TBH I rather liked how this was treated in Legends of Hermes.
First off, I have mainly experience with Theurgy - we're only now introducing a user of Spirit Magic, so some of this may not be as applicable to spirit magic as to Theurgy.
Firstly you have to consider how easily available you want "new" spirits to be.
If it takes a quest every time you want to invoke a new Daimon, just to get info enough to invoke it, you'll have a pile of stories, but they are likely to be ones only the theurgist cares about. If you can invent an Invoke spell for any Daimon you like, fewer stories will be generated, and the theurgist will be more powerful.
And how about True Names of spirits - a x5 multiplier for penetration! If I want to summon Chnobis the Serpent (might 30) I will need a spell no less than level 31, but if I want to call him in a single go, I need a spell of level 60 - and in either case, I'll need to penetrate a might of 30!
Having his True Name makes this much more easily done, but how avaiable are these True Names?
Do you need to know them to Invoke the Daimon at all? That would make sense, since it's one of the few ACs you can (reasonably) obtain to Daimons.
NB! Daimons do tend to gain power from being summoned and are generally hard to truely affect, so they actually have relevant reasons for not completely concealing their True Names.
The same consideration goes for Names of Power (and especially Invocation major mystery).
How easily can you obtain the Names of relevant powers? Is "Isis" good enough to let you call upon the powers of egyptian goddess of weather, magic, healing and childbirth? Or do you need to quest for a secret Name for her, known only to the secret remnants of her priesthood or perhaps written down in plain sight - but in a language no living person can read? Will you need a different secret Name for each of her aspects?
And how much bargaining will you have to do with each Daimon once called?
They gain from being summoned and so are likely to arrieve positively inclined towards the magus. They have very little to loose -even if the given avatar dies it barely affects the Daimon itself. But they might well still ask for services of various sorts.
In short, there is plenty of potential for stories here, but many of those stories might well matter very little to anyone but the Theurgist himself.
One other note of import is that Theurgy is not actually very powerful, compared to the main thrust of Hermetic Magic. Mainly it allows access to a few tricks not normally available to hermetics, but at the cost of a wider growth. With regards to the Titanoi of house Tytalus, the joke goes that Tytalii don't use Theurgy because it's an easy path to power. They use it because it's hard!
I guess it's because the "spirits" invoked don't seem to be fully fleshed out spirits, but rather spell effects we're calling spirits. That may just be my perception, however. They have a theoretical Might, but the invoked Spirit of the Pilum of Fire does not seem to interact with, say, a "Spirit's Eternal Oblivion" spell, or a Ring of Spirit Warding in the way a statted spirit would.
Mechanically, how has your theurge shone in play? Did anything seem under or overpowered? What neat tricks did you note?
It'd say that's an accurate perception. Spirit related magics in AM5 all suffer to a degree from being written by different authors with seemingly little coordination, but Theurgy in particular goes off in its own direction.
You mention the Spell Spirits of the Hermetic Theurgy virtue and I agree with you. They don't match any of the recognized categories for magical entities in other AM5 works. The same goes for the Names of Power and Synthemata. Both of these clearly are meant to fill the same or similar roles as the True Names used in other AM5 books, yet are inconsistent with them and poorly explained. Then there are Daimons. Mysteries implies that only through Pact spells can Magi interact with Daimons. I don't get the sense that other books follow this requirement.
I agree with you on it being closer to historical occult beliefs. That may be the problem; Ars in general is not interested (at least in Fifth Edition) in exploring the occult in any ways that conflict with conventional religion. There's no belief, no gnosticism, in AM5. You want Divine Word, talk to the Pope. It's not a bad way to do medieval wizards but it conflicts with any kind of historical theurgy. Incidentally, the kind of theurgy seemingly modeled in Mysteries isn't particularly medieval, but rather corresponds to beliefs from either Late Antiquity or the Renaissance.
Mechanically the overall Mystery package is quite weak for Hermetic Magi, who can already do most of what a Theurge does. The Spell Spirits have certain advantages but unless your Theurgic Order has a large library of lab texts you're going to find it harder to learn effects than a conventional Magus does.
I have plans to use Theurgy as the basis for the magic of a non-Bonisagus Greek order that casts spells only using rituals and theurgy, without access to formulaic and spontaneous magic. That's strictly non-canonical of course - canon says that only the Order of Hermes has hermetic magic.
Ah yes, Spell Spirits are a rather specal case, but generally (not always!) mechanically inferior unless you can claim a focus (in spirits or the like) or it otherwse gves you an advantage.
Quite agreed - almost.
Anyone can Invoke Daimons, the Theurgy virtue allows you to do so incrimentially, which is rather expensive though.
All true, except the bt about gnosticism. There are dark gnostics in RoP: tI, but they pale compared to the Philosophers of Rome (MtRE p.122ff)
Seconded. The tytalii do it not because it's easy, but because it's hard!
Spell Spirits are rather boring and not a terribly effective solution*
A tad underpowered, but probably not badly - the player is one of the most experience in the troupe and can play the lab-game better than most of the others, so it's a bit hard to say really.
Indeed. So very true. And here we have a MAJOR hook.
I would suspect that said Cult does not allow free access to all spells to all initiates in the cult. "Do you want new true names? Yeah, we have them, no problem. However, we have a small favor to ask of you before we handle down the scrolls..." Adventure time to open up a new cache of spells! And it can be something important to the cult, so nothing that is specially spirit-centric if you do not want it to be. So, enjoyment for all the troupe
Adventure time to open up a new cache of spells! 
And it can be something important to the cult, so nothing that is specially spirit-centric if you do not want it to be. So, enjoyment for all the troupe