Discipline priest is a really good healer class with many supportive abilities and the best possible way to keep your team alive: absorbing incoming damages. This is why they are a MUST in every 25 raid but also they are viable in 10 man raids along with a resto shaman or a holy paladin.
Shadow priest is a really good dps! It is a must-have for raids it has many utilities, buffs and supports and beyond these it has a really really good dps and mostly on fights where it counts, like Lich King or Professor Putricide. Mind Flay is a really good slow for Lich King valkyrs as well. As a summary with...
Holy priest is one of the most underrated healing classes in Wrath of the Lich King. But actually I believe Holy Priest is just an awesome healing class if played well and it is more fun than other healing classes to play.