It would be nice to see a poll option that expresses support for stores giving people discounts for using their own bags, but opposing using government force to ram through a one-size-fits-all solution as Ross Mirkarimi favors.
As for paper versus plastic, it is by no means clear that the former is more environmentally sound. The SF Weekly had a cover story digging into this issue a while back, which concluded that using paper is actually worse for the environment, and that the SF plastic bag ban was therefore counterproductive environmentally as well as economically.
Whatever the larger truths about paper versus plastic, I can say that in my case not having the plastic option has probably not done the environment any favors. I do much of my grocery shopping at Safeway, Trader Joe's, and Walgreens, all of which use paper bags whose handles tear off easily. Since I generally carry the groceries on the handlebars of my bicycle, that means I have to double-bag everything in order to avoid spilling groceries all over the street, when a single plastic bag might have sufficed.
As for using reusable bags, I do have a nice pair from the Libertarian Party of San Francisco which include a humorous message about the plastic bag ban (you can get your own for $12.99 at http://www.cafepress.com/lpsf.2537100). However I often don't plan my grocery shopping in advance, but just hit the store on the way home from somewhere else when I have the time and feel like taking time out to shop, and getting around by bike as I do, it's more of a burden to remember to bring reusable bags with me than for people who could just keep them in the car. So most of the time I just end up using more paper.
Unintended consequences of government intervention, as usual!