Recs.
Updated
Specs
Pros
Pro Compatible with D&D 3.x material
The D&D v3.x family was published under an Open Game License which encouraged third-party material and as a result provided an enormous pool of resources. And two D&D editions later, there is still a thriving market creating new 3.5-compatible resources.
Cons
Con Complexity
The DM has to have all of the rules memorized because there's a rule for everything. Want to control a frightened mount? There's a rule for that. Control a frightened mount? Rule. This is to say nothing of the myriad of feats designed for very specific situations (swearing at a favored enemy, mixing illusions with real weapons, meleeing with a bow, staying out of the way of friendly fire, etc.).
Con Detailed combat simulation slows down gameplay
Due to countless situational modifiers, players and DM alike have to put a great deal of thought into decisions about position, order of actions, etc., to the point that six seconds (one combat round) of "in-game time" frequently takes half an hour or more of play time.
Con Encourages character optimization outside of play
With so many options available to the player, and the almost unlimited ability to combine them, certain "character builds" are patently superior to others. Players who spend a lot of personal time poring over the rules can often create characters so powerful that players who don't optimize feel useless in games with those who do.