this works very well. we are in the process of doing this to replace some frame relay links and i have a pilot set up that is working great. over a cable connection (10 miles away) i get ping responses of about 24 ms versus 10ms over the 768k(12 miles away) frame pvc from a desktop to the call manager. From another remote site with a 768k pvc i get 35-40 ms (250 miles away)
The quality thru the tunnel is excellent.