We needed a place to store our bags in the centre of town and this place fitted the bill perfectly. We booked online and it cost around 13 Euro for two medium sized backpacks, for a day booking. This included a discount from airbnb.
It is located right next to the Old Town Square, right behind the Tyn Church.
Be aware though that your bags will not be locked away but just stored on the ground behind a desk. When you book the bags in, you will be given a number - in our case 21 - which you will be asked to photograph and use as proof when you pick them up.
The staff were friendly, helpful and we had no problem at all with the drop-off or pick up, although the fact that your bags are not locked away and are in sight of anyone who looks over the desk may be an issue to some people who have valuables in their luggage.