Are we talking about old guns, are we talking about old barrels that have been removed from guns and aren seperated from a gun? If we are speaking of just the barrel itself, I think I am the odd ball with regards to old barrels.
I will seek out old original barrels. If the bore is really, really bad I will have it reemed out and re-rifle it if the oportunity allows. If the bore is typical, used, groves are decent, I will use the barrel. If it's a iron barrel as opposed to a steel barrel I won't use it, but will use it as a display when I participate in a show like the one held at the Log Cabin Shop every October. These are great for showing the non believers what old barrels are, how they were made, the difference between steel and iron barrels, how barrels are made today....all sorts of stuff. Last year we had some people visit form China, (guest's of a professor from Ohio State) who watched us rifle a barrel and the old barrels as display were something they had never seen.
For using old barrels in a build, I see no problem. They were made to shoot and the propellent used was black powder. As long as the patch isn't torn apart and a spin can be put on the ball, the barrel will ussually shoot pretty good. As a rule, the barrels I have and have seen with a bore light pretty much have a twist in the neighborhood of around 1 in 50 to 1 in 60. That doesn't mean that that is concrete and faster twists in smaller calibers can be had, but as far as I have seen most of the twist rates lay within these measures.
I also notice that old barrels rifling have very deep narrow grooves, whereby new barrels have much wider grooves. In shooting old barrels thick patches are pretty much the norm with a grease as opposed to an oil or spit.
It used to be easy finding these at gun shows or old shops but now they are harder to find and when you do come upon them, they aren't cheap. I am willing to spend no more that 30 bucks, but I don't see the practicallity of spending 75 on up on one. But that's just my opinion.