An Ardent is what you get when you want to combine Jean Grey's powers with Storm's NO FUN ALLOWED personality - basically, a psychic Paladin.
I think we can safely say that the only reason they're in the third 4e Player's Handbook is so that there's enough Psionic classes to justify basically giving them their own book, like all those "Like a Fighter, but..." and "Like a Warlock, but..." Primal classes in the second one.
In combat Ardents function as psychic/psionic leaders. In 4e that amounts to a healer/(de)buffer who yells at his teammates to "do better" and "go hit that thing over there," while lightly slapping enemies on the chest to pretend like they're helping. Compared to other Leader classes Ardents appear to be more balanced. While they will never outclass a cleric at healing or a warlord at melee support, their psionic power source (augment-able at-will's) allows any one ardent build to be more versatile than most other class specializations. One should note that Ardents are melee leaders that start with poor armor proficiencies and defenses, making them one of the premier builds for polearms.
Addenda tend to break down into a couple types - striker/leaders with polearms or psionic healers. If oriented towards combat, Ardents work best when behind their teammates poking at enemies with a long stick - most of their arms-wills give buffs or rebuffs to adjacent creatures, but since their defenses are shitty, many Ardents like to hang back and focus on buffing the meatshields in front of them with temp hit points and buffs. On the other hand, Ardents make good healers once they get past 15 or so, when they stop messing around with temporary hit points and start dealing the real stuff. These guys have a daily that gives them another full set of actions, allowing them to daily-drop enough healing to bring a party back from the brink of death.
Flavor-wise, Ardents are psionic power users who were not smart enough to become Psions, and not good enough at fighting to become battleminds. Instead, they figured out they could read/manipulate people's emotions and decided to focus on that. Their mood also manifests in a cloud of colored energy called a Mantle that surrounds them and plays a role in combat. They fight by artificially bolstering allies' confidence and feeding enemies' fears and despair.
Role-playing-wise, Ardents are deeply disturbed people who lack any real personality of their own as they are constantly at the whim of the emotional consensus of the room. Crying at random people's funerals, wanting to join in on passing mobs, and getting involved in tavern dances when the party is trying to do something important are trademarks of the ardent class. The PHB3 even has a section dedicated to teaching ardent players how to play the class without annoying everyone else in the party. It is definitely worth pointing out that they display 6 of the necessary 5 signs to be diagnosed with Borderline Personality Disorder 
Overall they are one of the better options for a leader class displaying tremendous in-build customization. Provided you can get over the creepiness of having a party member that can read everyone else's emotions. If someone in your party thinks they are a good actor and feels the need to remind everyone of that on a regular basis, they will probably want to play an ardent.
|Dungeons & Dragons 4th Edition Classes|
|Player's Handbook 1:||Cleric - Fighter - Paladin - Ranger - Rogue - Warlock - Warlord - Wizard|
|Player's Handbook 2:||Avenger - Barbarian - Bard - Druid - Invoker - Shaman - Sorcerer - Warden|
|Player's Handbook 3:||Ardent - Battlemind - Monk - Psion - Runepriest - Seeker|
|Heroes of X:||Blackguard - Binder - Elementalist - Hexblade - Sha'ir - Vampire - Witch|
|Settings Book:||Artificer - Swordmage|
|Others:||Paragon Path - Epic Destiny|