"Whats the allure?" :hmm:
So far the answers you have gotten hit on a few of the reasons shooting a flintlock is attractive.
History:
The flintlock was the culmination of many different approaches to getting a gun to fire when the shooter wanted it to.
It was "the best" for over 300 years. From about 1530 thru 1830 the flintlock proved its worth, successively serving army's, pirates, explorers, hunters, trappers and settlers.
Even when the percussion cap ignition began to replace the flintlock the flintlock never totally lost favor among shooters because although the percussion system reduced the flintlocks weakness to wind and water and a small gain in ignition speed that didn't totally offset the difficulties of obtaining caps.
Satisfaction:
There is no denying the fact that the flintlock system is more demanding.
Using a rock and a piece of steel to light the black powder in the pan requires a good lock (and a little luck).
While just about anything that can pound a percussion cap will work to fire a percussion gun, a flintlock needs a cock and frizzen that will position the flint in such a way that the flints edge will produce a shower of sparks while shoving the frizzen out of the way, exposing the powder in the pan. It also must do this without greatly damaging the razor sharp edge of the flint.
Even with a good lock, the flint must be positioned correctly in the jaws of the cock to get the sparks and because the shooter is the one who installs the flint and makes this adjustment, his knowledge and talent becomes a integral part of the gun.
Learning how to prime the pan for the fastest ignition is another thing that must be done.
Not too little and not too much is the key here.
Also, wiping all traces of fouling out of the pan is needed after the gun is fired to keep the next prime dry.
With his good lock, a correctly adjusted flint and a properly primed pan the real test of the shooter will be measured. Shooting to the mark.
This requires several things.
The first is to ignore the flash in the pan.
With this happening just inches from his eyes he must not flinch or move in any way.
Although the cock and the fence at the back of the pan shields the shooter from the flash, it doesn't always shield his forehead from small bits of burning powder and learning to ignore this is not always easy.
There is also the time lag between pulling the trigger and the discharge of the gun.
While modern suppository guns will fire in one to three thousandths of a second, a flintlock usually requires seventy to eighty thousandths of a second after the trigger pull for the gun to fire.
During this time, the shooter must maintain the alignment of his guns sights on target. No movement of any kind can be made.
Adding to this time factor, more than a few times the flintlock shooter will find there is considerably more time needed for the shot to fire. Sometimes as much as a second.
He must be mentally prepared for this possible delay every time he shoots the gun.
Some call this "follow thru" but whatever it is called, it must be learned and practiced.
Now, I realize all of this doesn't make the flintlock sound too appealing but those who have learned to do all of these things successfully will have a inner satisfaction that can never be known by those who shoot the more modern guns.
There is another, often unmentioned benefit of learning to shoot a flintlock accurately.
The "follow thru" I mentioned will carry over to any gun you shoot.
Groups shot with percussion or suppository guns that once were measured in inches will become groups that are measured in fractions of an inch.
No. Flintlocks aren't for everyone, but for the person who wants to develop his skills to the highest level and have the total satisfaction of knowing that they and their gun did everything right, they are hard to beat.