First, let me explain what this item is, because I didn't really understand it until I got it. It is NOT like a pepper mill, where you fill up the outside and it feeds into the grinder and out the bottom. The box explains that the interior of the grinder can be used to store the spices, but they don't feed the grinder. To load the grinder, you have to remove the top, fill a small interior cylinder about 2/3 of the way, then put the top back on. At that point, cranking the handle turns the spices against the sharp slots in the bottom of the grinder, and ground spices fall through the slots.
Because the slots are fairly wide, I would say black peppercorns are about the smallest seed you could use with decent results. I bought it to use for grinding allspice, which are about pea-size, and it works great for that. Nutmeg, which are even bigger, would be perfect. Anything soft, like ginger, or small, like rosemary, it just isn't designed for. I bought this, and will keep it, because the regular peppermills I have tried just can't handle allspice - the seeds seem to be just too big to feed well. Now that I have it, I'm going to have to get some whole nutmegs. It would be interesting to try nuts and broken cinnamon sticks, as well.
The only issue I see so far is that I've been having a bit of trouble getting the lid seated firmly all the way around, but that may be because I filled the grind tube fairly high, so it's hard to push down firmly and evenly enough to turn the lid so all the plastic tabs catch. If this item fails, I suspect that those plastic tabs on the lid will be the reason.