Sacred Star of Milos provides an interesting side story in the FMA universe. Taking place
sometime after Ed and Al have discovered the true nature of the Philosopher's Stone.
While little in the movie implies that it must be fixed in either the original series' plotline or that of the Brotherhood series, the spirit and tenor of the film is much more that of the original. The themese are mature, the violence more brutal and the general feel more intense than Brotherhood. FMA fans that prefered Brotherhood to the original may find Milos a little intense and graphic. Fans of both series will likely like the movie.
While I have read numerous critiques of shortfalls in the art and production, it didn't ultimately take away from the experience for me. In many ways, Sacred Star is one of the better attempts at a stand alone movie from a famous series. These are hard to make well (i.e. in a way that doesn't seem trite or contradict aspects of the original series) and Sacred Star doesn't mess this up.
I was honestly a little disappointed in how little the regular stable of characters figured in. Winry, Riza, Mustang and Armstrong all appear, but none of them play significant roles.
But ultimately this is a fun watch. Lots of action and good alchemy. Definitely recommend it to fans of the franchise.
Oh, and one last note. The menu screen/ED song is terrible.