It has a slow swing speed, but makes up for this with high damage. With a well maintained super sledge, enough melee skill, and Critical Hits (Sneak or otherwise), you can kill most enemies (including super mutant masters and Enclave soldiers) with a single hit.
Super mutant overlord super sledge - Carried only by the rare melee-equipped super mutant overlord, this version has an object effect which inflicts an extra 25 damage per hit when used by non-player characters against the player. This damage ignores Damage Resistance and is independent of the condition of the weapon. This version is otherwise identical to the standard version of the weapon.
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
Discharge hammer - A cut version which inflicts additional electrical damage against robots.
End of information based on Fallout 3 cut content.
In the Operation: Anchorage add-on during the finale battle with General Jingwei, 2 power armor troopers can be seen wielding super sledges. These troopers can be disarmed. Their weapons disappear quickly when loose on the ground, but it is possible for the player or an NPC to pick them up.