This quote by the Korean official Yu Song-nyong, for example, is pretty damning:
In the 1592 invasion, everything was swept away. Within a fortnight or a month the cities and fortresses were lost, and everything in the eight directions had crumbled. Although it was [partly] due to there having been a century of peace and the people not being familiar with warfare that this happened, it was really because the Japanese had the use of muskets that could reach beyond several hundred paces, that always pierced what they struck, that came like the wind and the hail, and with which bows and arrows could not compare.
The Japanese were in agreement that their musketry was a great advantage. One of the Japanese commanders wrote home in 1592:
Please arrange to send us guns and ammunition. There is absolutely no use for spears. It is vital that you arrange somehow to obtain a number of guns. Furthermore, you should certainly see to it that those person departing [for Korea] understand this situation. The arrangements for guns should receive your closest attention.