Balanced PRCs: All of the old PRCs in the 3.5 DMG that were in the 3.0 DMG are balanced. i.e. Arcane Archer, Blackguard, Dwarven Defender, etc.
The PRCs that were added to the DMG in 3.5 are mostly balanced, although some of them are more powerful than the original 3.0 PRCs i.e. Archmage, Eldritch Warrrior, Mystic Theurge, etc.
PRCs in various other sources range all over the place. Some (especially in Dragon) may be overpowered. Others may be too weak. It just depends.
A good rule of thumb is that if a PRC has the major abilities of two classes, it is too powerful. i.e. Full BAB and Full spell casting = too powerful. This is one of the reasons the Mystic Theurge is overpowered, as it gives full clerical and wizard spellcasting. A more balanced cleric-wizard class is the Kiaransalee PRC in Player's Guide to Fearun, which in ten levels, gives 6 caster levels to both and some necromantic special abilities.
On the other hand, a PRC that gives alternative high level abilities for a class, is ok. For instance, a Barbarian 20 gets lot of abiliites from level 11 -20. A prc should be able to give equivalent specialized abiliities. The Dwarven Defender or the Blackguard are good examples of this, replacing the high level progressions of a Fighter or Paladin respectively.