The Hammer, is one of the most powerful weapon types in the game, in fact it can knock out giant Monsters, even after a few blows to the head. The Hammer, doesn't allow blocking but you can still run with it out and you can also charge it for a Super Pound or a Spinning Attack into a Golf Swing, all while moving. Hammers, are also good against fast moving or underground Monsters, as its focused on using it in a "Hit and Run" manner, which can be done by charging the hammer and striking at the right time, then escaping to charge it up again. When the Monster, gets KO'd, you can go for big damage with a Triple Pound, as the Monster, sits there helpless for a short time. If an enemy is struck in the head with the Side Swing (O Button), they will suffer alot of KO status damage, but very little raw damage, so use your best judgement, for using the Side Swing.
The only real downside to the Hammer is you must get close to the Monster, to hit them, so be cautious. Also when hunting in groups, you can anger other Hunters, if you keep knocking them away with the third hit of the Triple Pound or the Super Pound, etc.