I've been using them for a while now. I am really into functional fitness and use ketlebells, clubs, bodyweight exercises, etc. I bought a leather bulgarian bag from Escape Fitness for around £260. It is 22kg and was far too heavy for a first time user.
I also picked up a 12kg one from Dorset Kettlebells which is far better to use in terms of weight and cost, about £130 delivered. It is cheaper because it isn't leather but it makes no difference to be honest. Also try Wolverson for vinyl ones.
The exercises are fun and varied. I feel a real difference in my core strength and they give a fantastic conditioning workout. See YouTube for videos on how to use them. It is nice to have a bit of a change in your routine, and they create brutal explosive power and endurance in the legs which should translate well into your running. Hope this helps.