I settled on Baggu, ordering 6 bags -- 3 in red, 3 in striped blue and white. I'm pretty happy with them, so here is a quick review. The Baggu bags meet all my requirements:
lightweight (made of nylon) but they feel sturdy;
small profile (you stuff them into a compact 5x5 inch pouch) but large volume;
they are washable; and
I tested out the bags on a trip to Central Market and found that $70 worth of groceries -- including a gallon of milk, 5 pounds of flour, a whole chicken, lots of produce and various other things -- fit comfortably into 3 bags. The heaviest bags felt very secure and comfortable in my hand and on my shoulder. The handles are long enough that I could sling a couple of bags onto my shoulder. When it came time to fold the bags back up, I had no problem stuffing them back in their pouches. I also liked the shape of the striped bag enough to use it as a waterproof carryall this weekend (it seemed appropriately patriotic, with my red flats) and I say it can totally moonlight as a casual bag for schlepping around on the weekends.
One final point: Since the Baggu bag is not attached in any way to the pouch, I worry that I will lose the pouch. I mulled over this. I did find another brand of bags, Envirosax, which can be rolled up and secured like a sleeping bag but, in the end, I was more drawn to the Baggu design and colors. If you are a fan of graphic prints, however, lots of people really like the Envirosax.