Not a bag review, per se, but the yarn guide is such an important part of the Yarn Stuff Sack for us yarn crafters that I thought it deserved its own review.
I have two Yarn Stuff Sacks, a large solar I ordered late last year and a small steel that I just got packaged with my new Little Swift.
The solar has the (second generation?) guide, which is a strip of fabric that folds over and closes with a snap. You can see it in the pictures of the large solar on the Yarn Stuff Sack page, particularly pictures i and j.
The steel, though, has a caribiner like clip that I have not been able to find any pictures of. I assume that it is a new (third generation?) design, but feel free to correct me if I am wrong. I hope that I am not, however, because it is awesome.
The snap guide is good and I don't hesitate to use it, but the clip is faster to get the yarn in and out of and the yarn moves through it more smoothly. The snap guide holds the yarn perpendicular to the height of the bag, so you have to pull it a bit sideways rather than straight up, while the clip doesn't distort the vertical line.
I would also feel more comfortable putting multiple strands in it at one time, say if I were doing colorwork. The snap guide is a tight enough fit that I would worry a bit about felting if I had multiple strands of heavy yarn running through it.
In summary, one thumb up to the snap guide and two thumbs up to the clip guide. Here's hoping the clip guide is to stay!