Well, I've used a diamond "steel" for seven or eight years, and none of my knives has disappeared. But, it's household use, not professional. I've got several waterstones, but don't use them very often for the knives. (I do woodworking and have lots of other edges to sharpen with the waterstones.)
If they DO remove some steel, I'd guess that a diamond steel would wear away less knife than alternating a regular steel with a stone.
My favorite woodworking supply store, www.leevalley.com
has a nice diamond steel with a 12" blade (or whatever you would call it- it's round) by DMT, a company specializing in diamond-grit sharpening equipment. Lee Valley also has all kinds of waterstones and such. They also have an interesting array of kitchen tools, such as Microplanes and the like.
The DMT steel is about $35.
Stay sharp! :bounce: