Is the compression zipper easy to close or the fabric tend to caught up in the zipper a lot?

I only got the fabric caught in the zipper once and it was easy to get out, but I also wouldn't describe them as easy to close. If you are not putting much in them, why bother with a compression cube at all? It took some grunt work to get the zippers shut if I actually filled the bags.

How is it a set if it contains one bag? (i.e., a group of things that belong together)

They have sets of 5 bags in different sizes plus the laundry bag. I love them.. It used to drive me crazy to get my suitcase all messed up looking for something that off course was right on the bottom. It just makes things easier.

Do you need a vacuum to compress them?

No, you do not. Keep in mind, they do not compress completely like the ones that you do use a vacuum on, and no, you can't use a vacuum because part of the bag is mesh.

What are the weights for each of these cubes?

The entire set weighs 447g/16.7 oz. Individual weights are: Large: 137g/4.8 oz. Medium: 93g/3.2 oz. Small 78g/2.5 oz. Extra-Small: 59g/2.1 oz. Slim 70g/2.5 oz. Laundry bag: 37g/1.3 oz.