Excellent discussion going on here, ladies and gents! I am so impressed by the deep questions and answers present here. Our community makes me proud!
Ruben, I'd like to bring up a couple of things to add to this good topic.
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You might like to read my Moz Post http://moz.com/blog/local-landing-pages-guide which covers the concept of building out pages for various locations in some depth.
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Citations are a big part of the picture - absolutely! They serve 2 main functions which are a) to help customers find you on a wide variety of platforms and b) to build a consistent, trustworthy body of information about your business on the web, which is then crawled by search engines, causing them to have confidence in the cluster of data they have about your business which can then increase your chances of being considered a relevant answer for certain queries.
So, it's a 1-2 punch kind of thing, building citations. There are direct benefits in a customer finding your listing on a site like Superpages and indirect benefits in the consistency and prevalence of your citations helping search engines to feel confident about the validity and prominence of your business.
This being said, it's my belief and understanding that it takes time for validity/prominence to actually start affecting rank. Every local business platform has its own schedule for taking a citation from zero to live, and then I have to think about search engine bots picking this up and stirring it around in the bucket in which they've got all my other business data. Then, I have to take into account whether my citation building has, in fact, surpassed what my direct local competitors have done or has simply brought me on par with them, meaning citation building will not be the difference-maker I'd love it to be, because my toughest competitors have done the same thing I have (meaning, I'm going to have to find some other way to distance myself from the field).
In sum, citation building is a must-do for any local business that doesn't want to fall behind in their niche, but it takes time to see the positive effects from it and the level of positivity is going to directly relate to the stiffness of the competition. You are in a tough market - law in major cities, which I would consider one of the most difficult ranking environments. Yes, you've got to get those citations in good shape, but my honest appraisal of this is that you are going to have to go way beyond these basics to surpass what is doubtless a highly active/ heavily marketed competitor base. The usual advice of build excellent content, implement on-page Local SEO, build citations, earn reviews is going to be common to all your competitors', so finding something that they're not all already doing may be necessary to see yourself rise in the SERPs by dint of some extra creative, superior effort.
Hope these thoughts are helpful! Glad you brought up this topic, and I hope once Moz Local has had a few more months to bake in the oven, we'll be able to publish some case studies that prove a correlation between participation and positive results!