Occultists are basically Harry Dresden, the class. They're built on the same chassis as the Magus- that is, 3/4ths BAB, 6-level (spontaneous) spellcasting, and good Fortitude and Reflex saves. However, they differ greatly in use.
First, the Occultist gets access to Implements, which are objects that he casts his spells through. Every few levels, he gains another implement, and gains one spell known of every spell level he can cast that belongs to that implement's school of magic. Second, they get Focus Powers and Resonant Powers based off of their implements. At first level, you get two implements, their resonant powers (passive abilities that typically buff you in some way), and the basic focus powers (which are generally useful abilities, like summoning a monster or shooting a laser). These powers are fueled through your "Mental Focus" pool, which is equal to your level plus your intelligence modifier.
Uniquely, at the start of each day, when you refresh your spells, you get to allocate your Mental Focus through your implements. The more mental focus you have invested in an implement, the stronger its resonant power will be, and the more you can use its focus powers. You CAN opt to use a focus power without investing focus in the implement, but it costs twice as much.
Occultists also get a hodge-podge of abilities as they level up, ranging from always-on detect magic and aura sight, to being able to draw magic circles a la-Fullmetal Alchemist that allow him to use Protection from [Alignment] spells, and later on, bind outsiders. At 20th level, you supercharge one of your implements.
The Occultist solidly rests at Tier 3. It's nowhere near as powerful as a full 9-level spellcaster, but it gets the job done. It also has a lot of staying power, because it doesn't need to cast buffs using its limited spell slots. They also have a great deal of versatility- they gain a large number of spells known all throughout their career, and because their focus powers allow them to have the most useful spells as powers, they can afford to invest in other spells.
Also earns creativity points for the Tome Eater archetype, which allows you to eat books to regain mental focus.
|The Classes of Pathfinder|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|