In my opinion it is a bad idea whether you are running on or off road. On the road the risk of being flattened by a car is increased. Off road, you are making yourself much more vulnerable to attack, and still have to watch out for mountain bikes.
I also find that running with music completely destroys my balance and running style (such as it is). About a year ago I bought a little MP3 player to run with off road, but only used it a couple of times. I found myself constantly looking over my shoulder to see if I was about to be overtaken by mountain bikes; I had to remove an earpiece every time anyone said anything to me (which happened quite often on that route); and I ran about 15 sec/mile slower.