Funny your talking about ketones, my boyfriend is working on that as his master degree project.
Ketones are pretty natural and is a source of energy the same way the glucose would be. However, is you HAVE glucose in your blood, the body will take the glucose first instead of the ketones.
The phenomen of ketosis is mostly seen in the human body in starving conditions, when you are low on glucose so your body will generate ketones to use as a source of energy. for exemple, diabetics, who can't take a huge glucose intake, are known to have a much bigger ketosis than non-diabetics people.
So my opinion is that it probably wouldn't be harmful (as not toxic) but it probably wouldn't do much to your metabolism, because your body would still use the glucose you got from your food intake as an energy source.
Hope it helps