Mine love theirs. Out of six pigs, only one doesn't like to sit in the hammock.
But I didn't buy a hammock. I used to make a towel tent for them to hide under and when I would clean them, I would clip the bottom corner of the tent to the lid of the cage so I could clean behind it. One day I forgot to unclip it and I found Carlton just hanging out, happy as a clam on it. So since then, I have experimented with different ways to hang various pieces of material and such. I think I have found the best method.
I clip a dish towel to the cage lid. It hangs down in a U-shape. When they jump in, it doesn't have much swing and them seem to like how it cradles them. I fold the whole short end of the towel over about an inch and clip the whole end with binder clips (5 or 6 of them, each end) to the lid, not just the corners, and I think that helps it not to sway. I use a terrycloth dish towel instead of fleece because the terrycloth is thick and textured, so the fabric won't slide out of the binder clips the way fleece might.
Here's a pic of Errol enjoying the hammock: