These are by some distance the best corner cushions I have found in 5 years of looking - and I have tried MANY others!
They are made of thick soft foam rubber or plastic, not the plain plastic all the others are made of, and they actually stay in place, once stuck - even when my 15 month old son tries to remove them! These are the only corner cushions I have ever tried where the double sided tape actually works.
However, they do have one faily significant drawback - they are not exactly invisible. As well as being the best corner cushions by some way, they are also the largest by some way too, being about 2.25" (5.5cm) long on each edge, and about 1.25" (3cm) wide, and the foam rubber is about 3/8" (1cm) thick (they are bigger than they look in the picture). They are also a not particularly attractive beige colour. So, if you want to protect your little darlings' heads, but don't want huge bits of beige foam rubber stuck on each corner of your table, these may not be the product for you.
But, if you're serious about safety, these are the only corner cushions to buy, the others simply don't work, or don't stay in place (or both!).
If you are worried about the look of these corner cushions, I'd recommend the Lindam ones as a good second best. These are smaller and made of translucent plastic, so are far less noticeable, but still give quite a good cushioning effect as they are made with two layers of soft plastic, sort of hinged together. My 15 month old bounces off them quite happily. The double sided tape that comes with them is absolute rubbish though (the corners fall off by themselves!), so you'll need to buy something better to stick them up with. I would recommend Sellotape Outdoor Sticky Fixers, or the 3M strips that come with the Clevama corners - these are the only two double sided tape strips that I found actually work. The Outdoor ones are thickish pads and black (and quite pricey), so you might prefer the 3M ones (transparent tape), but I don't know if you can buy them separately (and the black ones actually look quite cool with the Lindam corners, anyway).
One last point - I nearly knocked off a star for the time it took to deliver these corners - 8 days from order to delivery. Not really acceptable.