I too have lost several of these ball bearings taking them apart and I had them for over a year before someone at an Op in OK told me that you can reset them without taking them apart by pushing the firing button really hard with a pencil or something. He was right and I felt like a fool. Also, if you do loose ball bearings, any hardware store with a decent selection of ball bearings will have ones that will fit. They're usually the smallest ones they have but they will seal. O-rings and springs can also be replaced at a well equipped True-Value dealer.
They're versatile, cheap, and no huge loss if you lose them. I plan on buying another set when they restock them.
Can be underloaded increasing range
Can be left unloaded to propel nerf rockets in longer barrel launchers.
ABS, can break if dropped on concrete
internals easily lost if not disassembled carefully
relatively low powered