I've never personally done one, but while they're cool and I'd think it was a cool project to build and shoot....
But I'd never use one for hunting. At least here, where muzzleloader season can often be a battle with rain and moisture keeping things dry, I'd take a modern inline design any day of the week for a number of reasons (better cap protection, removable breach, etc). I capped on a cow elk this year, and never got a chance to punch my tag before the season ran out, sucks.