Elements of Magic (EOM) has to be the best d20 magic system mod in existence. It allows for a free form spell casting, while on the surface, may be slow, in play time, it isn't much slower then the basic system in the regular D&D 3rd.
There are three new core classes introduced in the system which pretty much either suppliments the game or replaces the spell caster types already in the basic system. They are the Mage, Task Mage and Warrior Mage.
The Mage is the class that would end up replacing The Sorcerer, Wizard and Druid. Clerics Can use the Mage or the Warrior Mage depending whether they are primarily Fighers with divine powers or Spell users with some combat training. Mages start out with the most 'Spell Points', and 'Lists', which means, magically, per level, they have a broader spell design base and a greater number of spells used per day.
The Task Mage is sort of the fill in for the Bard. It's basically a Skill based class with a magic kicker.
The Warrior Mage is the fill in for the Paladin or the Ranger and also perhaps a Warrior Priest who does better at arms then their spell casting.
Instead of pre designed spells in the book, there are lists. And there is an extensive set of lists when you put all the 'verbs' and 'nouns' together. For example, one list is a Healing list, but you have to specify what you can heal when choosing a list, 'Heal Humanoids' is one as is 'Heal Metal'. There is also Evocation, Charm, Abjure as well as another dozen or so.
Spell casters normally have a few spells that they have as 'preferred' or memorized so it only takes a normal casting round to use. When a caster wants to design and cast a spell on the fly, it takes two rounds to cast. Which game wise works out, since the player can tell the DM the general purpose of the spell they are working on for their first round, then get the specifics during the time of the rest of the players/monsters are going, then do a normal casting the second round.
It might be mentioned that in EOM that there is no difference between Arcane and Divine Magic. So, unless your DM specifically restricts it, your Mage can use healing magic. Personally, I don't have a problem with this. Where Divine is Faith Healing, Mages actually alter the body to mend the wound.
Also, there are a few dozen new Feats as well as alterations in Feats that already exist. Meta magic works a little different with spell points and Item creation has been overhauled completely to use the EOM system. They are generally computable with the old system, though the costs tend to be different.
Pros: I like the system and have used it with success. It fits my concept of real magic users and allows me to get away from the seemingly artificial 'memorized' spell system that D&D uses.
Cons: It can slow down play considerably when the plaer using it is not on the ball. Also, it can be overwhelming to some with just the shear flexibility that they will encounter.