Martin,
I like that sharpener very much. My kids all carry that model in their kit. I carry two:
Extra Coarse/Coarse
and Fine/Extra Fine
But I would be as well-equipped with the Coarse/Fine model you asked about.
If a tool can sharpen a blade, it can probably blunt it as well, but I wouldn't be concerned about that. Practice sharpening with a kitchen knife. Follow the directions - not heavy pressure or you risk damaging the dianmond matrix slightly. Use dry or with water. I would prefer a solid backing for the matrix on the fine and extra fine, but that's just me - these are fantastic portable sharpening devices.
Regards,
Tom