If you REALLY need to keep things dry, like lying in several inches of water all day dry, dont be stupid enough to use a cuben bag. They are not real drybags. They dont seal anywhere as good as real drybag with smooth rubber seal with stiffener does.
They work fine for keeping gear dry backpacking though. They are stuffsacks with a fold-down closure. I have some issues with them termed "dry bags" and sold as such, they are not. Anyone buying cuben gear though should be smart enough to know this.
I have several Zpacks, and one MLD.
I much prefer the Zpacks. Its a better made sack by far.
I dont even use the MLD. The buckle was attached to the bag with a strip of cuben tape, which slid and bunched up on the buckle attachement,(who would thing adhesive would soften up when it gets HOT outside ?) but since its sticky, you cant straighten it out. Horrible idea. You can see the "creep" of the tape as well.
Consider what might happen to something like this left out in the sun.
Zpacks has it right. Sew it, seal with tape. These items get a workout stuffing things into them.