I adore mine. The potatoes come out of the microwave so much better than if you don't use one. And much quicker than the oven (plus no need to heat the house up in the summer!)
We never leave it unattended, and microwave in one minute intervals, then flip the bag, and do another minute (usually takes 6 minutes for up to 2 potatoes).
It doesn't have a closure- the top just folds over, and the steam holds in fine. I didn't make it, but I know you have to use the exact materials called for- all cotton thread/fabric, special batting.