By merging themselves with other people, the user can grow tremendously in size and strength. Inorganic objects can also be seemingly included in the fusion, as the merged form of the Charlotte Decuplets, who each wield a normal-sized scythe, carried a massive scythe.
The fruit seems to offer no benefit to the user if they don't have other people around to merge with them, and it is unknown if they can merge with unwilling partners. Other than that, it has shown no particular weaknesses aside from the standard Devil Fruit weaknesses. 
Newshi mainly uses his fruit's powers to merge with his other nine decuplet siblings, turning into a giant creature wielding a very large scythe, which increases their fighting capabilities significantly.
Gattai (合体,Gattai?, literally meaning "Combination"): The Decuplets stand in formation next to each other and Newshi activates his power to fuse the ten of them into a massive figure armed with a giant scythe. It was first used while fighting the Vinsmoke Family on Cacao Island.