Clan Halls are very very useful, and their usefulness is based on where their locations are... generally, the higher level areas have better equiped clan halls.
Clan Halls (this is assuming you pay for all their services)
--Provide buffs. Clan Halls have nearly an infinite amount of mana to provide a random set of buffs-- the higher the clan hall's level, the better the buffs it provides.
--Provide Mana regen-- Sitting in a clan hall improves your mana regen by a ton.
--Economic Functions-- All clan halls have purchasable Clan Hall SOES, Clan Hall BSOEs, Clan Shields, etc.
--Teleports: Clan Halls will teleport you a number of useful places both in-town and out-of-town. Its like having an event gatekeeper.
--Exp Bonus: You can choose to "Return to Clan Hall" instead of "Return to Town". Depending on the clan hall and how much you are willing to spend for it, clan halls will give a % back, as if it were a +40 level healer rez. The Gludin Clan Hall my clan owns gives back a 40% rez if we choose to return to it instead of to town.
Also great when red. You can SoE to clan hall or BSoE to clan hall while red. The NPC in the clan hall will sell to you, buff you, and teleport you while you are red.