Benefits of Membership:
All Fighters Guildhalls look the same (though in the desert areas to the south they are frequently called "Fighter Trainers"). They are square three-story towers and all offer the same services (training, weapon repairs, quests and a place to sleep). The Fighter Trainers guildhalls count as holy places, so vampires and PC's with certain items should be careful about resting in them.
Fighters Guild quests come in a few basic varieties: kill some critter which got into someones house, kill beasties which are plaguing the area (harpies, giants, orcs, etc.), kill a werecritter (werewolf or wereboar) which is causing problems. The Fighters Guild also provides a couple of unique quests involving finding assassins and the infamous Lord Kavar quest. Some Fighters Guild quests will also help increase your rep with Merchants and Commoners.