Product review (5 stars): Great product
I have both the round 12" and the 9x13" lids, and Amazon keeps putting whatever I type in the review on both product pages, so I'm going to review both of them here, since they were both excellent, 5 star products

*great seal that keeps food fresh
*easy to remove and put on
*easy cleaning (no grooves like snap on lids)
*can be used on bowls smaller than 12"/ most 9x13 pans

-won't stay on if you turn the bowl upside down
-limited colors
-potential to stain from dark/red things (like most plastic and silicone)

I was a little late to the silicone party, but now that I have started using it, I love it. The bottom of each is flat and smooth, except for where the handle is. They kind of suction cup the container/bowl and form an air tight seal, which helps to keep food fresh longer. My banana bread stayed fresh for about a week. You would think that it would be hard to remove; however, it peals right off when you pull up on the tab. My family has arthritis and carpal tunnel, so the idea of an easy on/easy off lid is appealing to me. The one thing I don't like is that it is the limited color selection, everything else in my kitchen is green.

The 9x13:
I ordered this lid for my old metal 9x13 pan that never had a lid; however, when I received it, I happened to have a 9x13 plastic container on my counter, so I popped off the plastic lid, which didn't seem to be air tight, and set the lid on top. I wasn't impressed because it just seemed to sit there. Then I tried to take it off by pulling up on the handle. The whole container, which was full of banana bread, lifted right off the counter. I wouldn't say that I would carry it around the kitchen that way since you never know when the weight will get to be too much and the seal will break, dropping your stuff in the floor, but it's nice to know how good of a seal it has. I tried it on both my metal pan and my pyrex glass pan, which has a lid but what the hey!. It was able to pick up both pans! That's a great seal. I didn't think that it was going to work with my metal pan at first because it seemed to barely cover the squared off corners, but to my delighted surprise, it did. Since I already had food in the plastic container, I decided to try leaving it on that one. It kept my banana bread loaves moist and fresh tasting for about a week.

The 12":
I got this one after seeing how great the 9x13 was. I tested it on my 12 glass bowl and a plastic container, and it fit both. I was making homemade frosting in with my stand mix, and that bowl (metal) doesn't have a lid, so while the cake was baking, I stored my frosting in the refrigerator in that bowl with the 12" lid. The bowl is only 8.5" but it still worked well.