DisruptiveI don't own any Neoballs, but I've got loads of Buckyballs (at least two sets of every color they produced), and I've spent a lot of time playing with them.
That said... I don't expect either gold or nickel will get your hands dirty, *until* the plating starts to flake off, at which point the metals inside will start rubbing off a little on your hands.
The magnetic forces will can cause them to hit each other with a fair bit of force, and over time it is inevitable that the plating starts to break off. Exactly how long this takes will depend on how and how much you play with them... and how good/thick the plating is, I suppose.