I picked up a flintlock one at a gun show 1 or 2 years ago for about $100 with a western style holster. It was a decent deal IMO, though largely because the holster was pretty nice (though it didn't work well with it). I had no intention of keeping it, but did a little tweaking to the lock mortise since the lock didn't seat fully to the barrel, and gave it to a friend (with the holster) at cost, who has done a lot of shooting with it - his first BP gun.
They're ok. As far as build quality, many are kits, so that will depend on the builder. They generally shoot fine, but have a lot of extra wood on them for my taste. You can find the same kits available as CVAs sometimes.