Overall, this is a great product. I work as a commercial seaplane pilot in the Bahamas, and wanted to upgrade my pilot bag to something waterproof, high visibility, and easy to carry/use.
I ended up buying two of these bags: One was the 10L Hi-Vis Orange bag, and the other was the 20L Hi-Vis Yellow bag. I'll review both, starting with the 10L.

10L Bag:
One thing you have to keep in mind with these bags is that they require 3-4 folds before you buckle the top closed to be effectively waterproof. What this means is that the bags are not as tall as you would think just looking at them in the package. I haven't actually attempted to measure the size of the bag when it is closed to see if it is actually 10L, but I would assume the overall space is reduced when closed (especially depending on how many folds you put in before closing it). With this being said, I can easily recommend the 10L for 3-4 medium sized handheld items (which I will specify now).
For myself, I stored an aviation headset bag (measures about 6x4x10 [LxWxH]), a pilot logbook (6x2x12), my passport booklet, sunscreen, and an ipad mini. It was snug, but I could still get 3 rolls and latch the bag shut without any problems. Initially this was okay, but I wanted a little more room so that I could bring extra items like extra clothes, bandages, water, etc. as my days are long and spent in the sun.
Now, I carry the 10L bag with me (Folded up nicely in my 20L bag) as an extra buoy if I have to cut an anchor line. These bags are air tight when closed properly and will hold any residual air inside once closed. All I have to do now is blow a little air in the bag, roll it, latch it, and it is a buoy or flotation device.
The 10L bag is great if you don't have TOO much stuff to carry, but I'd caution you to go with the larger option if you are unsure and aren't confined to a small space for the bag.

20L Bag:
The 20L bag functions the same way as the 10L, but supplies twice the space. I have plenty of room for all of the items mentioned above, and some. An added benefit is that the 20L comes with 2 straps instead of one, so I can wear it as a backpack when walking around the islands. Beyond that, there really isn't much difference at all between the bags.

Pros:
-The bags are simple to use and versatile
-The color options are great, especially if your use for the bag may end up with the bag falling overboard
-The bags will float when sealed properly
-No water has been found in my bags yet (but I'd still add extra caution and seal anything you absolutely can't get wet in another plastic bag as well)

Cons:
-Folding the bag over eliminates some space, which means you need to plan your space utilization carefully if you want a waterproof bag!
-The bag is not designed to be submerged for more than a few seconds... it is waterproof when floating (which is what it does). If you force it underwater for too long, the pressure of the water may force some liquid into the bag

Overall, I'd say this is a great, CHEAP, and simple product. Take it from someone who's out on the ocean all day: it works!

I bought the 20L to prepare for floating in the summer and used it a week later for kayaking. The size works great. Fit a towel, water bottle, wallet, keys, shirt, shorts, and still had some room to spare. Great size for a day trip and comfortable to wear with the wide straps.

The product seemed to work fine but the blue dye from the bag rubbed off and stained my clothes and skin. The dye wouldn't completely wash out of my clothes so they're now ruined. Not cool.

Great dry bag for kayaking! Fits a change of clothes, towel, and other accessories. I can even clip my Bluetooth speaker to the strap!

Great buy. Bought for floating the river. Water did not go inside. We got the 30ml it was bigger than expected everyone of course wanted to add things inside we fit many shirts and 2 beach towls in with room to spare. Only thing was we got black and it did get hot inside but we had warm towls at the end :)