5,500 hours on our Challenger and only one leaking seal since we bought it at 1,800 hours.  The transmission on this tractor has cost us a lot of money but everything related to the tracks has been extremely reliable and we are spread out over 40 miles so it sees plenty of road travel.  Tracks do have some limitations to where I wouldn't want to have all of our big tractors on tracks but at the same time after having one I wouldn't want to not have at least one track tractor around.