I saw those too, but at $12+ each, didn't feel like experimenting with them this year. The experiment I AM running is with reusable grocery store bags -- the type you can generally get at the checkout for $1. They're made of a polypropylene weave, similar to the growbags. I've read that they typically only last a season or so because of UV breakdown, but that's enough for me at the moment. I suppose you could get the "double walled" feature by placing one bag inside the other -- might also help with the UV issue as the outer bag should protect the inner bag from the sun. (hmmm... maybe I'll go get a couple more bags ;))

emgardener, the 'grow bags' I have used had an impervious plastic base and a fabric-like wall. They were used for growing trees for later transplanting. Roots would grow through the sidewall and break off as they thickened. This resulted in a profusion of rootlets inside the bag. The advantage is that when transplanted the tree had a mass of functioning feeding roots which mitigated the 'shock' of transplanting. It worked very well for this purpose. On a related note, I once used brown paper bags to create a "container" to hold a composted mix and moisture in place for tomato seedlings transferred to ground planting. The roots grew through the paper bags and I had a good crop of tomatoes.

Last year there was a whole thread on this. Some even make their own out of landscape cloth which has the UV factor in it. One poster made hers from Eco-Felt.

Smart Pots is a maker of these, and here are a couple others:

http://www.rootmaker.com/products_containers.php#jump
http://www.fanntum.com/advantages.htm