The biggest complaint about feet are cracked feet and this is not covered by warranty and you have to buy new replacement feet. Rubber feet are the worst. If you are buying a rebounder and don't want that tire store smell in your home, I would avoid rubber feet and look for Polymer feet. Polymer material even though the feet look similar to the rubber feet (see pictures below), they don't rub off on the floor.
Polymer also lasts the longest. I have been using Polymer feet since I heard about them and never had to replace them.
The rebounders most people get have rubber feet.
Rubber feet are very cheap to buy and crack over time. Once the rubber starts splitting in half you have to change them or one leg will be shorter than the others and the rebounder will be off center.
Polymer feet will never mark up the floors are scratch. They should hold up for many more years than rubber feet. I get about a third the amount of calls from people wanting new feet replacements than broken springs. The Cellerciser is the only one that I never get these complaints about.
The Cellerciser has the Polymer feet. The Bellicon rebounder has some type of blend that also doesn't rub off on the floor. The white feet are the ones with the proprietary blend. I still don't know what it is made of but they don't have the rubber smell which is good. Rubber does have a better grip on the floor but wears out quicker. Polymer slightly less but with the Cellerciser, the legs have a wobble so it makes up for the difference much better
The white rebounder feet can start to look dirty but at least they don't mark up the floor. The grip is not as good with the white material. Rubber grips the best, then Polymer, then the white feet whatever they are made of.
If you are going to get feet that are not rubber and have a chemical sensitivity, make sure the legs have a wobble to them and are not the screw in type. Screw in legs are very rigid and don't "move" with you when you do dynamic exercises.