Superior Will lets you save against daze or stun effects that last until the end of the enemy's next turn, which can be a huge advantage. If your DM uses lots of enemies with daze and stun effects, this sounds like a good investment, particularly if you don't have bonuses to your saves (e.g. the feats Stubborn Survivor and Human Perseverance). So even if you don't make your saves as easily as other characters, the circlet will give you two chances against daze and stun effects a save will normally end.
On the other hand, if you do have bonuses to your saves, I think the circlet is a little redundant. There may be more attractive Head slot items depending on your build.
To answer the original question, a character with Superior Will who wears a circlet of Arkhosia may make one saving throw against a daze or stun effect at the start of his turn even if the effect doesn't normally end on a save.
If the effect does normally end on a save and the character fails the save at the start of the turn, he may make another saving throw against the effect at the end of his turn as normal.
If the effect does not normally end on a save, e.g. it lasts until the end of the enemy's next turn, the character does not get to make another saving throw against the effect at the end of his turn.
