Some of the more modern ones have replaceable internals - I've done it on an XT jobby. The whole guts unthread from the wire side and you just drop a replacement in. Not an very cost effective but can save you the arse ache of a wheel rebuild (esp. if you're running discs)
I absolutely don't have the time, tools or skills for that. I will give the hub away for rebuild so one of you weirdos (read: proper people) can repair it
It's a secondhand shimano one, I was informed that they weren't. Anyway all the local mechanics started slowly backing away when I brought it up