These are a very helpful idea and do work at keeping little hands out of cupboards, but they can be tricky to fit. Fitting to a large cupboard door is ok, you just need to make sure you align the two parts correctly, but when I was fitting them to smaller cupboards or drawers I found it quite fiddly as there is no room to get the screwdriver onto the screw heads whilst holding the clips (it would also help if the supplied screws were self tapping).
When fitted they mostly work well; on drawers they are a little awkward to open, but then this is the point of them. However, if your little one is as strong as my boy then he/she may be able to pull hard enough that the clip simply lets go; it doesn't break, but simply bends out of the way. Although it did take to the age of 18 months and he is quite big, so they should mostly be fine.