Nothing sticks like PopSockets. You get one, you put it on the case and if you want a different design you can simply change the plastic. They are incredibly stable and the folding part is so hard that it can withstand the weight of the mobile without hurting the fingers.
I have been using PopSockets for over 3 years.
I have never tried to remove them and I have no opinion if the adhesive leaves residue.
If you change covers frequently or use them like me as a fidgeting device by putting on and taking off the plastic cover, they have a lifespan of 1-1.5 years because the folding part loosens afterwards.
Beware of imitations, it's not worth getting the two-euro ones because they will come off.