We have seen dramatic increases in site traffic and better native search positioning for our clients, but we wanted to be sure that the SEO value of  sites that had built great positioning through years of business would not be lost when switching to Magento and using 301 redirects.  As Magento Professional Partners, we now have access to a whole new world of resources through Varien, so we asked them about what results they had seen.  Here is a portion of their response, straight from one of Varien's many clients:

"Yesterday I attended a very interesting presentation of the new Jack Wolfskin online shop (Globetrotter in Germany) based on Magento (yet community, next year enterprise). They seemed to be very happy with your product and increased traffic (+ customers) by 600% in 10 months just by moving to Magento (due to better SEO capabilities)."

While the results are not necessarily typical, we have seen dramatic results just by implementing Magento with 301 redirects.  Long story short, if you are thinking about switching to Magento and you are concerned with the impact of the change, don't be.  The greatest concern you should have is the clients you are missing by not using the robust platform of Magento to enhance your SEO.