We've been running a 963 since 2001, it was a $3,000 head that the dealer thought was junk, we rebuilt it an have been running 1,200-1,600 acres a year since then with very little in season downtime.  The weak spots on those heads are the lower gathering chain sprockets but they are cheap and easy to replace.  We have also broke the driveshaft on ours a couple of times but it never happened until we started running it on our 2377 so I think it's mostly due to the fact that we are running it on a combine that's got about 100 more hp than the head was designed for.