The Spinfusor, the Stormhammer, the Disc Launcher, no matter what you call it, this is the bread and butter weapon. It is the weapon that essentially made Tribes what it is (that and jet packs). You have not mastered Tribes until you have mastered the Spinfusor.
The spinfusor is so unique and powerful because it is a gun that shoots explosive frisbees, as opposed to the traditional bullet. It adds a totally different element to gameplay as you have to lead the disc as opposed to shotting directly at the target.