I used to buy Marks & Spencers' version and was happy with them: these are a lot more efficient and I understand how some people might hurt themselves if they are rubbing too enthusiastically. You need to be gentle, especially when they are brand new! Only increase the pressure if it feels confortable. Do not attempt to remove all the hair on the first day. Even those that have not come off at first, will be erradicated on the 2d or 3d day (or even whilst rubbing against your clothes, because they will have been greatly weakened). As well, make sure you start a new mitt on the legs and not on the arms... (as I did!...)
I only use them on my legs and arms.
On the first day, I worked the mitts for 1 or 2 mn per half-leg, even less on the arms. Afterwards, I use them a few seconds everyday as maintenance, or whenever the hair becomes noticable again and starts bothering me.
Despite having dark/strong hair and, very pale/sensitive skin, I never have to wax or shave anymore.