Last year when I moved from Pittsburgh to Philly (miss yinz!) I biked out and took the Ghost Town Trail for its length. Here's the route I took btw:
Most of the trail has a slight grade traveling eastward, as you head towards Altoona and the Allegheny Mountains. The furnaces are really cool sights to see. I blogged along the trip, here is the section with photos from the Ghost Town Trail: http://bikepa.tumblr.com/page/8
(Hit 'older' at the bottom to see more from the trail)
The trail west of the Blairsville is really amazing to see, as it goes above the Conemaugh River only by a few feet, as the river is damed up there. There was a detour that you had to go over a hill for, as the trail wasnt yet completed and it was a bit more rugged, but pretty neat. Also along this part is a closed railroad tunnel, that was part of a never-completed railroad due to the Panic of 1857 (http://bikepa.tumblr.com/page/10).
Overall, I really enjoyed that part and the furnaces. Throughout the trails, it's a peculiar combination of natural beauty and mine waste that has tore up patches of land.