The simple fact that many tire manufacturers will not honor their warranty if there is evidence of beads or other foreign objects inside the tire is enough for me to avoid using them. I am also not very convinced of their effectiveness from a scientific perspective, but as I am not interested in voiding the warranty on any of my bike tires, I'm not likely to try them anytime soon.
One of the longest and most inflammatory threads on the BMWMOA forum was started by this discussion of dynabeads a year or two ago..