There are many advantages to using a Shrink Bag, starting with the wide mouth design.
The wide mouth is the most obvious feature of this group of stuff sacks (Shrink Bag 20, 40 and 80). All three sizes have the same features - the difference is in volume. But we'll focus here on the Shrink Bag 20 because we recently used it on a trip.
So yes, the wide mouth is great. It's easier to load and easier to find stuff once you get to camp. As shown in the above photo, contents can be readily viewed and easily found. 20 liters is about the right size for a backpacking or mountaineering trip and is big enough for most sleeping bags. It is also a great size for clothing. Also, the white coating makes for a relatively bright interior - easier to find things than in darker bags.
Also built into Shrink Bag is a purge valve. This makes removing air from the stuffed contents much easier than sacks without purge valves. When the contents is stowed and the roll-top is closed, simply open the purge valve, kneel or sit on the sack, and push the air out and shrink the size of your load substantially (hence the name Shrink Bag). To ensure that clothing or other items don't block the valve, inserting a finger in the valve works well and keeps the air flowing. It's surprising how much air can be removed from clothing or a sleeping bag by this method. The bag will go from looking like a grape to looking very much like a raisin!
Another convenient feature built into all three sizes of Shrink Bag is the tail-like QuickStrap. To quickly and easily open the sack, unclip the quick release buckle, grab the QuickStrap, and pull up. Because the strap is rolled up in the folds of fabric at the top, it forces a rapid unrolling of those folds when pulled against the weight of the stuffed contents. Bam! Your gear is completely visible and accessible in a flash.
One final note about Shrink Bags - they are waterproof. The seams are factory taped and both the purge valve and the roll top create waterproof closures. One important note: no roll-top stuff sack is submersible - the pressure will allow water to find its way into the folds. But in any other watery application (bottom of a canoe, sitting outside a tent in the rain, inside a non-waterproof pack) these guys are good to go.