To start, this is really great quality and the material is very strong, yet flexible. I am extremely happy with this in all ways. Here is what I love about this:
1. It actually is waterproof. Some waterproof blankets advertise as waterproof, but in actuality are not. This one definitely is!
2. It is the most versatile of any waterproof blanket I could find - being sand resistant and waterproof, I appreciate that is has multiple functions. (The top won't catch sand like regular blanket or towel would, but is not crunchy like a tarp would be.) This also makes it much easier to clean.
3. It is very thoughtfully designed so that it is extremely lightweight to carry and packable. I have the large and the medium size, both fold up to fit inside a backpack. Huge plus for those that want to travel with this, or even if you are just carrying gear and want to minimize bulk.
If you are thinking about what size fits you best, this might help: While the site advertises the medium at up to 4 people and the large at up to 8 people, it would be a really tight to fit the maximum advertised capacity of either. The medium is probably best suited for 2 people and the large is probably best suited for 4 people. My only other suggestion would be to think about making an XL size that is square - possibly 84"x84", which would make a square out of the longest side of the large. Great product overall, would highly recommend!