Bovy finished, presented at MPIA Galaxy Coffee, and submitted to the ApJ his paper on dynamical inference based on Galactic masers today. He shows that the Reid et al masers can be used to constrain the Galactic potential, provided that sufficient prior information is provided about the position of the Sun in the Galaxy and about symmetries in the maser phase-space distribution function. The issue is that the masers do not represent an angle-mixed population, but nor can they be assumed to travel on circular orbits (they are observed not to). So you have to assume something in order to get the dynamical inference going. They are measured very precisely, so in principle they could have a lot to say about the Milky Way. However, most of the information in the observations is used to infer things about the maser phase-space distribution function, the parameters of which are (for our purposes) nuisance parameters and must be marginalized away. Look for it on the arXiv on Monday.