I've only used the DMT 8K a couple of times, but I think I can still tackle your question...
If you have a Norton, the 8K diamond hone will be redundant. If not, it could serve as your intermediate to fine hone.
The consensus seems to be that the DMT 8K cuts fast but leaves a harsh edge for an 8K hone. You could shave off of it, but other hones would be better. Since it cuts so quickly, you might be able to get away with setting a bevel on a DMT 1200 grit and then jumping right to the 8K before going to a final polish, such as a pasted strop or coticule.
The 8K would not be useful for anything else, such as lapping other hones.
I'm a DMT fanatic and I have not yet broken down for the 8K. It just doesn't seem very useful to me.
Looking forward to hearing from guys with more experience on this specific hone...
Josh