I'm gonna post the following to RPG.net's review database, I thought I'd post it here for comment first.
The Mysteries Revised Edition (TMRE) is a supplement for Ars Magica Fifth Edition, 140 pages of new secrets of magic and the cults that use them. The presented mysteries are intended to serve as a carrot (or stick) for all magi; this is not a book about the four Mystery Cult Houses of the Order of Hermes (if you're looking for that, seek out Houses of Hermes: Mystery Cults).
Prolegomena
The book details the mechanics of acquiring Mystery Virtues through Initiation, which are basically centered around the strategy of sacrificing time, effort, and resources in return for Virtues that allow the magus to do things a vanilla Hermetic magus cannot. In this way a vanilla Hermetic magus will probably be stronger, in some respects, than one steeped in mysteries – but will lack the ability to do the many interesting things the Mysteries allow. The mechanics also socially involve the aspirant with the cult. This is a good structure, with flexible and story-driven mechanics, that should aid in constructing mystery cults and running them in-game.
A problem with mystery cults is that the stories involving them focus very much on the interaction of the magus with the rest of the cult, leaving little room for the rest of the gaming group to participate in the story. Much of the second chapter is spent in dealing with such issues, amongst other storyguide advice related to mystery cults. Although there is no ultimate solution to this problem, I think the advantages Mystery Cults bring to the saga's variety, character involvement, motivation, and development far outweigh their problems. Even if you find such solo stories to be problematic, I would advise you to read TMRE as there is much here that can be used without such stories (as the chapter rightly notes).
The Mysteries
Having dispensed with the preliminaries, TMRE moves on to describe a rich plethora of new virtues and mystery cults (36 virtues in about 110 pages). The new magics are truly exhaustive, covering everything from astrological benefits to spellcasting to achieving (pagan) divinity. Instead of going through them, I will make some general observations.
Overall, the new magics are interesting, enriching the setting with new ways to do the previously undoable and with cults and magical systems of thought. They allow the SG to make Hermetic NPCs that wield strange, unpredictable powers with interesting flavor and often a secretive cult behind them, complete with interesting politics, cosmological and magical worldview, and even more mysterious powers. These can be used as mentors to guide the PCs into a rich story of personal development, but also as a foil and antagonists. I think any saga can stand to benefit from including material from TMRE into it, and any storyguide can gleam some story ideas from perusing it.
The Bad
TMRE is not without its problems. The things that stood out for me were:
- The book is very poorly edited. I've seen many spelling and typographic mistakes. Errata was also needed on issues of consequence (for example, without errata immortal magi under TMRE fairly rapidly cease to learn), which should have been caught in playtesting.
- The rules are sometime poorly written. For example, on page 30 it says “To cast a Performance Duration spell, the magus must succeed in a roll... of Chatacteristic...+Ability against an Ease Factor of 3.†That almost implies that this roll replaces the normal spellcasting roll, although this is not (I think) the intent. As is often the case in Ars Magica, the rules are not written clearly and accurately enough.
- Things are sometimes fairly limited in actual scope, even if the fluff or preconceptions hint otherwise. I would have expected Divination to allow one to glimpse the future, for example; instead diviners can only guess at it by analyzing the present, essentially employing normal Intellego magic. Many virtues' benefits are subtle, however, and at first glance they may appear to be weaker than they really are.
- Some mysteries allow to do things that you may consider Hermetic magic perfectly capable of doing without delving into Mysteries. Perhaps you think any magus should be able to calculate the astrologically favorable times to engage in certain actions; perhaps you think any magus should be able to invent spells that allow him to enter a dream. TMRE would still be useful, regardless, by detailing and giving ideas on such matters.
These are all pretty minor concerns, however. TMRE is a very useful work.
The Cults
There are 10 Mystery Cults described in TMRE. They are intended to serve as examples, not as actually existing cults. As they serve as good examples of what TMRE's Mysteries allow, I've decided to explain a little about each.
- The Order of the Green Cockerel is an alchemical society of magi exploring the new science of alchemy. The society encourages mundane alchemical research, but its main focus is in teaching its magi how to purify themselves and reach alchemical perfection. This not-so-secret society teaches its magi to harness the powers resonant in nature to better extract and purify raw vis, and use it to purify themselves. It is led by the Secret Master, who alone has achieved complete purity and, with it, immortality.
- The Magoi of the Star are astrologers, using the power of the stars to enhance their magic, and even prophesy. They follow a strange, heretical, variant of Christianity based on the writings of Origen, and consider themselves the inheritors of the Three Magi.
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The Volehsbnii Mechtateli are 'dream witches' that play with the dreams of others as an artist plays with paint. The members of this highly secretive society, meeting only in the dream world, have great power over dreams and nightmares, and no moral restrictions whatsoever.
4. The Neo-Mercurians are a society that toils to rebuild and discover the Mercurian ways of magic of old. It practices ritualized, pagan, magic. Its priests call upon the power of the old pagan gods, conduct Rituals at nearly no cost, and wield some unusual magics to do with roads and spell mastery. - The Legion of Mithras is a secretive organization devoted to heroic defense of the Order. It encourages the ideals of Greek heroism, and in the higher echelons its magi are privy to secret ancient magic.
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The Philosophers of Rome are a highly-secretive and loose Gnostic group of proud, show-offish, licentious heretics. These charismatic cultists encourage themselves to bask in their own glory, to live and be admired as gods amongst men, and ultimately to achieve true divinity by such worship.
7. The Children of Hermes are a growing cult reminiscent of modern televangelism. Led by a highly charismatic figure claiming to be a reincarnation of Hermes himself, and by his consort who he claims is Cleopatra, they claim to open up a new era of magic and open the path to all Mysteries of magic. Its members pass on initiation rites stolen from other cults up the cult's chain, and Hermes' revelations down it.
8. The Mystic Fraternity of Samos is a group of Pythagorean cultists who master the magic of harmony, geometry, and numbers. Although membership is openly acknowledged, and middle-ranking cult members even teach their more mundane arts openly, the cult's powers are kept in secret. They are somewhat famous for their huge architectural works, sorcerous music, and mastery over all manner of spirits. - The Disciples of the Worm is a secret and abhorred society focusing on learning the secrets of death and beyond. If you meddle with it, expect to hear of graves, ancient ghosts and ancient magic, bound spirits and awakened guardians. Some whisper it is led by the ghosts of long-dead magi, living dead maintaining a mockery of life by the cult's hideous magic.
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The Knights of the Green Stone are a group of magi knights, searching for a legendary stone in a quest similar to the mundane's search for a holy grail.
Of course not all these cults will exist in any given saga, and even if some do the book strongly encourages you to modify them.
Conclusion
TMRE presents a wide assortment of interesting ideas for new magics and new cults, allowing the storyguide and troupe to dream up magi wielding unique powers. TMRE exposes new ideas in magical cosmology, provides tools to enrich the politics of the setting and hidden secrets to be ominously revealed. It manages to provide tools that should enrich any saga. While the structure does have some problems (such as concerning overly-individualistic stories, or segregating perfectly reasonable Hermetic effects into cult-only mysteries), these are relatively minor. I'd give the book 4 out of 5 in terms of substance.
In terms of style, TMRE leaves much to be desired. The rules are often poorly written, and I found the writing only rarely engaging. The structure of the book is of scattered 'tools' to be picked and chosen, which I found probably useful but somewhat frustrating to read. The poor quality of editing also weighs in on this point. I'm afraid TMRE scores only 2/5 in style.
Sadly TMRE had only one playtesting pass - two rounds are the norm now, and i think things have got much better. My only complaint would be that I do speecifically state in the fluff on divination that divination derives from the present situation, and does not allow a glimpse of the future - in fact I think I go on about it fo rthe best part of a page. 
