Its not harmful. But you will not able to refill it more than 3-4 times, cartridge will be damaged by then
It is not harmful
It is not harmful, only if you do it properly. secondly refill it when their is still a bit of ink left in your cartridge. because your cartridge has less chances of working again if you refill it when it is dry from the circuit (bottom).
The major chains of stores are going to have refilling centers for ink cartridges soon.
The quality of the quality of refilling.