My contrarian points:
1. They're not picking on the left. Many bloggers speculate that contrarians like picking on the left but not on the right. I don't know about other contrarian authors, but Levitt and Dubner's first book talked a lot about how legalizing abortion reduced crime. Even the current book sympathizes with legalizing prostitution, something that's mostly libertarian/glibertarian, but still probably supported more by the left.
2. Geoengineering with sulfate aerosols is cheap. I've seen a number of assertions that they've left out costs of the stratospheric shield and exaggerated the costs of mitigation. It doesn't matter. As long as you simply examine the cost to cool a certain amount globally, without regard to distribution of heat reduction, or any other side effect, then the shield has to be cheaper than 80-95% greenhouse gas reductions.
3. A limited stratospheric shield for the Arctic might be worth the risks. I believe RealClimate wrote about this idea in a very tentative but not-completely-dismissive tone, but I can't find the link. (Superfreakonomics also mentions it, but they seem not extremely interested.) The Arctic is experiencing so much warming and the atmosphere is somewhat isolated there, so it might be possible, maybe, to counteract the enhanced warming there without the repercussions elsewhere, if the aerosol particles actually stay in that region. It couldn't be done for a few decades until the ozone-depleting chemicals are gone from the stratosphere, but it shouldn't be done anytime soon anyway - this is an act of desperation. (UPDATE: John Mashey points to high-albedo, artificial rafts as a potentially useful "band-aid" approach for the loss of polar ice, and possibly ice elsewhere.)
4. Somewhat less contrarian: geoengineering on a global scale needs to be researched and kept in mind as a last resort for the worst-case scenarios. If 50 years from now we find ourselves on a trajectory to a Lovelock-type scenario involving deaths of billions, or even a somewhat less-bad outcome, then smogging up the earth's stratosphere and hoping for the best might be worth rolling the dice. I think this position isn't all that unusual, even if Al Gore might disagree with it. By contrast, people like James Annan and William Connolley probably shouldn't be interested in geoengineering because they don't think the worst-case scenarios are plausible.
Okay, so much for being generous to Superfreakonomics. I'll just add two points that haven't been discussed too much. First, the authors have occasionally defended the stratospheric shield approach as a complement and not a substitute for carbon mitigation. But if that's the case, why do they keep talking about how much cheaper it is? And the failure as far as I can tell to admit in the book that we need to drastically cut emissions means they're trying to have it both ways - show a radical solution to those not paying close attention, and then running it backwards when caught.
Second, they seem to think it would be easy to determine that that shield isn't worth its side effects and turn it off if appropriate. I'm not so sure. If a powerful country or number of countries reached the point where they felt it was in their interest to smog the stratosphere, I expect they'd be very resistant to arguments that an ongoing drought in Africa means they should cut it out and drastically reduce carbon emissions instead. Just as we see massive foot-dragging today to the idea that we're causing warming, I expect a lot of self-interested denial would occur as to whether it's necessary to gut the shield.
One other point that has been mentioned but is still worth highlighting is this excellent Michael Tobis piece on removing carbon versus interfering with sunlight - two radically different approaches.
Unrelated bonus blogging: soldier fly composting. I get flies in my worm bin anyway, so this might be the solution.
UPDATE: forgot to mention that a critical comment I submitted to the Freakonomics blog was never published. Maybe it just fell through the Internet cracks.