The Factotum is designed around Inspiration Points. you have a number of IP that replenishes each encounter. Those IP are used to cast spells, provide bonuses to attacks, saves, AC, damage, skill checks, heal, turn undead, and so on. As you level up, the pool of IP grows.
3/4 BAB, simple and martial weapons, use light armor and shields with no penalty to arcane casting.
You also eventually add your Intelligence score to Strength and Dexterity checks, find a way to overcome SR and DR, even cheat death once per day. One of the most powerful abilities would be granting yourself an extra standard action in a turn by spending IP.
You gain the ability to cast a very limited number of Arcane spells per day drawn from the Sorcerer/Wizard list, maxing out at access to level 7 arcane spells at class level 18, permitting you to prepare one spell at your highest level (7) and seven other spells up to level 6 by class level 18.
Every skill is on your class list. This allows for access to some obscure Class skills, like Lucid Dreaming, Autohypnosis, Use Psionic Device and iaijutsu focus.
At level 19, you get the ability to possess three Extraordinary abilities of any base class for a minute. While this is supposed to be an incredible ability, I'll be honest, I so far haven't found how to make it very useful.
The Factotum is able to accomplish nearly everything, almost as good as a melee combatant or arcane caster, a limited number of times. He can skill monkey better than most skill monkeys, a limited number of times. He can serve as a backup healer a few times a day.
Your IP pool is pretty limited, meaning you can do a handful of pretty cool things each encounter, and then you're little more than a glorified Expert NPC class. A creative and prepared player will prepare for this, and act wisely. A more foolish player will do some amazing things, than run and hide until the battle is over.
These are some of the things that make the Factotum a good class. I don't, however, think they are a broken class. I'm sure it can be used in any number of creative builds. This class permits a creative player to have a wide variety of options at any given moment.
Factotum Handbook
Another Factotum Handbook