We picked one up a few months ago - they work.

no crisping but nicely baked.


This simple bag creates a "just right" baked potato in your microwave. The
inside is well done, yet moist, and the skin remains tender.

The bag will cook one to four medium sized potatoes at once.



Instructions:

Simply wash and dry your Potatoes. DO NOT PRICK!!
Wrap the potatoes in a paper towel and place in bag.
Microwave as instructed by the microwave manufacturer. Time will vary per
microwave.
When serving, do not cut with knife, simply use a fork. potatoes will be
very fluffy.
Let the bag air dry. ENJOY!!

http://tipnut.com/make-your-own-potato-baker-bags/

Free pattern/tutorial to make your own potato bags found on the web archive
he Potato Baker Bags
Caution: Please do not leave unattended and make sure to use only 100%
cotton materials. From a comment below: If your microwave is too hot to use
the bags, either turn down in temperature or don't use the bags (sparking,
small burn hole problems in the fabric, or can cause fire).

Dimitri