I would summarise it as quite good. But it still depends on a thousands of factors like the type, the quality of the materials and the smith,thickness, and even pure luck. And its a huge topic with many unanswered questions and ongoing debates.

A well made mail armour would protect the wearer against all cuts, and would be decent against thrusts. We see references for mail both holding against punishment, and being penetrated. For example during the battle of Acre, the leader of the templars got accused of cowardice for fleeing the field. He then lifted his arm to reveal that an arrow was lodged all the way to the flechettes in his armpit. On the other hand we have an account from Usamah ibn Munquidh from late XI-th century, where he tells a story of running through a Frankish knight with a lance to little effect, because the knight was protected by mail.

Plate armour would often even better protection - where mail is flexible, plate is rigid, so it handles the brute force of the blow much better. And with some use of geometry can redirect the force of many of the blows one would receive. However, plate still has gaps where a weapon could find its way, and in some places the armour is thinner. So where, say the front of the breastplate would withstand basically everything, the side or the visor would be weaker. You can ask (well you cant, he is a bit dead) Henry V, who got wounded by an arrow, that either penetrated his visor, went through his eye slit, or he had his visor open.

As to the use of weapons and armour - what do you mean by saying "as we can imagine"? Most certainly weapons and armour were not used as portrayed in most movies (and fantasy novels). By the 15-th century Europe had several old and well developed martial arts traditions that covered fighting with and without armour and with all the weapons of the day. In most cases fighting with armour one on one or in skirmish would be about going around it -finding the gaps between the armour and using locks and grappling to disable the opponent. Of course, on the battlefield it gets much more complicated. I will not comment on proper battlefield use, because I do not have the knowledge on that. And I am not sure there are many people who actually do - the fencing manuals we have from the period very rarely deal with battlefield situations.