These are one of those tools that every woman needs. It's a rare woman who doesn't own a pair of beautiful but horribly uncomfortable shoes. These are great because they give you some cushioning in the sole of heels that are otherwise like walking on cardboard.
Having said that, there are some downsides. They are sticky - which is good for making them stay in place, but means they can take a while to get in the right spot (but once there, they stay there). The other downside is that if you have wide feet like me, this adds to the bulk in the front part of your foot, making tight shoes tighter. Having said that, this could be an advantage if you have skinny feet (I wouldn't know, not having that problem!).
End result? If you don't have them, you probably need them in your life!
I don't wear heels that often, my work shoes are a pair of wedges that have about a 3cm heel. The cushions make the highest pair of heels a lot more bearable and I find they are much more comfortable to walk in as well as it gives your toes a better grip. However, if you have sweaty feet/toes, you may find these a bit slippery. Being reusable, it makes these little pads incredibly cost effective.
Tips: Make sure you try on any shoes you are looking at buying with the pads inserted as you will find any shoes that are 'just right' become 'too small'.