Mine love them, in all sizes. I use small ones to help my aged girls climb more easily into things. They love hiding under the big one in the playpen. And, of course, they love chewing them. They last for ages & ages.
As for the walnut things, the big ones with the walnut are great and they last a long time. The tiny ones are not worth the money. I once bought the tiny one, and my rats demolished it within a day.