Comfortable, and the 50 cm inserts fit nicely in my 45 cm covers. They are made by Inner Green, but, for 4 inserts, it was ~$7 cheaper to buy them from Inidgo Lane than buying them direct! Shipping was fast, and they were well packaged and clean.
I chose these because I hope they are a more ethical option than virgin PET, I'm mildly allergic to feathers, and corn based stuffing is prohibitively expensive. The inserts have zips so if they go flat over time you could add a bit more stuffing. The filling looks identical to the cheap (virgin PET) toy stuffing you can buy at Spotlight.
I'm very happy with my cushion inserts.