How effective were they? Obviously they weren't anything like Uzis or howitzers, but they must have been a considerable improvement over the ones first used in combat.
God is subtle, but He is not malicious. (Albert Einstein)
Obviously they improved as time went on. The early ones were very heavy, too heavy to hold or use without a stand. They were slow to reload and left the firer highly vulnerable so they needed pikemen to defend them.
That said, they were surprisingly acurate and could kill at ranges over 100 yards. They were easily capable of piercing even the heavy cavalary armour at close range and thinner armour at longer range.
Also, because of how they worked (gunpowder ignited by a "match") they were often rendered useless in damp or wet weather.
All correct, to which might be added that these firearms probably failed to discharge about half the time.
The obvious advantage of guns is that it enables ordinary men to become long distance killers without the years of training needed with a bow. The ability to fill an army with farmers and the like armed with guns was a huge leap. In previous centuries these 'ordinary' soldiers often went into battle armed with rudimentary weapons like clubs and pitchforks.
It's an interesting fact that in his (excellent) meisterwerk Longbow: A Social and Military History, Robert Hardy speculates that a company of long (or war)bowmen fighting a company of impressed men, or militia, ie: not professional full-time infantry, would probably have won the engagement right up unto the start of the Great War (that's 1914 for us, BTW). Their rate of fire, range, and accuracy far outmatched anything firearms could manage. Not bad for a weapon that won us the Battle of Crécy in 1346!.. The disadvantage, of course, is that it took a lifetime of training to become proficient-starting as young as seven, boys would be taught how to draw. There are plenty of skeletons in museums dotted all over the country showing how hours and hours of practice left the bowman with a distorted physiognomy, owing to the enormous drawing strength required (200lb). It was considered so important, that Longshanks (Edward I of England) banned all sports on Sundays, with the exception of archery, to encourage all Englishmen to practice...
I've seen things you people wouldn't believe...