No, you definitely wouldn't use it to get across the state quickly. You'd use it to see that part of Pennsylvania. As for pre-Interstate days, I'm sure it was as good a way across as anything else, but that's relative to the fact that one didn't just cross entire states in a matter of hours then.
I'd recommend any number of parallel state routes then - can't be much slower, definitely more scenic because YOU'RE IN FRONT.
Not a whole lot of those to choose from, but I agree they're worth looking into for scenic purposes. In fact, I'm fond of quadrant routes because they interfere the least with the landscape, have little traffic, and are lots of fun to drive!
That said, I guess I've just had better luck with US 6...some slow traffic, sure, but at least as much of that is RV's and powder-blue Mercury Sables (geezermobiles) as big-rigs, and what truck traffic there is seems mostly of the local variety, hauling loads of gravel or milk from the crusher or dairy farm, so it eventually turns off. (Only to be replaced by a lumbering tractor...)
I'd put it this way: US 6 across PA is mostly an un-upgraded US highway (unlike most of US 22, for example), so while it doesn't move traffic as efficiently as an upgraded highway, it offers a more appealing experience of the landscape it traverses as a result. And while it's far enough from both I-80 and I-86 to attract its share of local traffic, it's close enough to them that it doesn't get much long-haul use. So given all that, I'd say conditions are about what you'd expect them to be.