I use a .54 Colonial as a teaching rifle for the "intro to flintlocks" class I do. I'd have to check to be certain, but I think about 150 students ranging in age from 8 to 91, have shot it. Typically (depends a bit on class size) the students shoot ~10 shots, and we hang around after class and shoot til it gets dark or folks are tired. No one has really had a problem with the weight of the rifle.
One of my shooting partners had a problem with the length of the rifle. (Thanks to the late unpleasantness in the Mid-East he's missing some limbs; on bad days he shoots from a chair.) He bought a Woodsrunner (also .54), we did some re-working on the trigger etc to suit him, and he has managed it for as many as 80 shots in a day.
At the August shoot over in Iowegia--the First (Possibly-annual-but-maybe-not) Fall Invitational--someone sponsored a "Kibler match", where for $5 you got to shoot a Nightmare target at 50 yards; winner won a WR kit but all contestants had to shoot a Kibler rifle. There were maybe 4 Kibler rifles there Friday, but some folks went home and got theirs to shoot the match on Saturday. What with that, and with rifles being borrowed, I think the match had ~30 entries.
What I found interesting was that a number of people who had Kiblers weren't otherwise shooting them as an offhand match rifle (I don't either--I think this was the first match I've shot that rifle in). Many stated they preferred more muzzle weight. On the other hand, the girl who won the SMR in the spring match said she'd been "lifting weights" because the rifle is heavy.
But a good time was had by all, which is why we were there.