Everything you can grow yourself is that much you will know is organic and will not have to buy from the store. This is a neat idea and no costs out for containers that would need to be stored. Most of the bags of potting or top soil have plenty of nutrients to grow in for at least 4 to 6 months.
For folks with limited space for gardening this idea from Care2 looks like it would work on balconies or patios.
To grow deep rooted items you could try stacking the bags two high and cutting the bottom of the top one open. You could grow a lot of salad greens though and herbs.
A Super-Simple Way to Grow Food: Start a Bag Garden