The sponge is... Well, it cannot hold much water so you will be filling it up quite often.
The case can, however, easily be filled with either orbeez or silica gel depending on if you want to add or remove moisture.
Some people use a soap bar for this as a DIY solution, which is bulkier . This is smaller.
One small negative though is that the lid is on the loose side so if you have anything else than the sponge in it it might be a good idea you secure it a little better. IT might even be, that you are unlucky and the sponge falls out and come in contact with your instrument.
It has not happened to me, at least not yet, but it is a small concern.