They do NOT get priest spells. (Ack.)
Class: Dispel. Soul or Demons are banished, no XP.
Class: Soul Strike: Can hit ghosts, and ignores armor of everything else? Half armor? Not sure.
Class: Musical Instruments: They can use special musical items
Class: Barrier: Erect a barrier that weakens attacks. High priority.
Mastery: Physical attack up (226.9%)
Mastery: Old Fashioned: Equip seals instead of clothes to attack up to 8 times in a row.
Special EX Skill: Sacred Lot: Switch XP and Gold values gained at the end of the fight are changed (switched?)
Starting Class: C. No class featured are passed on when changing classes.
Ending Class: A. They do great damage, and with Old Fashioned, they can do a lot of damage AND do a ton of status effects as well.
No Class Change: C. No magic, innocent gear isn't worth it, etc.
They're basically a glass cannon like a wizard. They are absurdly powerful against ghosts, undead, and demons. With Barrier now having a number of uses, they're not as super great as they were. In Original (and 2 / 3?) they had an ability called "Oro Curse" that caused them to do a straight double damage on top of everything else to Demons / Undead / Ghosts / etc... that's gone. Still, almost every one of their weapons do double damage to those, so, woo.
Old Fashioned basically makes the class. They can do an absurd amount of damage. They need the EX Skill "Swallow Killing" which lets them ignore the Samurai's counterattack ability, though, since enemies can be Samurais now. Using Alchemy to make their weapons powerful against other races is good too. Also, most enemies aren't immune to sleep (but are to say, Beheading or Petrification) so adding Sleep to your Shaman weapons works well.
Supposedly in the postgame dungeon there's Shaman gear that's super effective against EVERYTHING, so... yeah. And while they focus on their special weapons, nothing says they can't use clubs and whips in their hands (and equip their seals to their heads / feet / etc).