Probably depends on where in the sharpening set of steps the stone lies. If you are replacing a coarser grit stone or a stone used for flattening, I'd say diamond stones are great and worth the money. However, for the last finishing steps 4000, 8000, and up, I'd go with some sort of waterstone like a Sharpton Glass Stone. I'd guess there will be other recommendations, probably equally as good.
I'm not the expert on sharpening, of course, but I think water stones like the Sharpton do a better job for the final honing steps. Diamond stones are great for doing the hard cutting work of the initial steps of flattening the back of a chisel or plane blade and/or cutting a new primary bevel, but don't seem to give quite the same final finish as a water stone.
Just an opinion based on some use of diamond versus water stones in the final steps.