Update: upon watching some truly atrocious anime, I've decided to bump my rating of this up two notches, mainly for the art and characters. Make no mistake, I still support everything else in this review.
Having finished FMA '03 (FMA), Conqueror of Shamballa (COS), and FMA Brotherhood (FMAB), it was only natural to move on to Sacred Star of Milos (SSM). While I still have my gripes with the ending of FMA and COS, they provided a reasonable ending to that story, if not a completely satisfying one. As COS was a film, I had certain expectations of it regarding production quality, and I was not disappointed; Sacred Star of Milos, however, did not rise to any of those expectations, to put it mildly.
Story-wise, this movie is filler; while it uses some concepts and characters from FMAB, it doesn't influence the series in any way and is never referenced in FMAB. Consequently, most of the suspense is dulled as you know none of the core characters suffer any permanent damage — unfortunately, this means they don't learn anything new either, and the characters and events introduced here are forgotten as if they never existed. However, this is not necessarily bad, as maintaining the status quo is a requirement of midquels.
Story - 3/10
If you've seen anything FMA-related in the past, the plot will hold no secrets or suspense; rather, some plot points are annoyingly convenient and border on lazy writing - for example: the existence of so many philosopher's stones, and the fact that everyone and their mother — including non-alchemists — seems to know what human transmutation does.
As for the pacing, it felt like most of the scenes of the movie were stuck together without any kind of cohesive transition, and the storytelling was pretty basic: this is the part where we're shown the horrors of war, this is the part where we find out the bad guys aren't actually bad, this is the part where the heroine is in a pinch but gets saved at the last minute (several times!) and so on. Thus, the progression of the movie feels less organic and more like rushing to get to the next checkpoint on the list, resulting in a soulless story.
Characters - 3/10
The characters come across as shadows of their former selves. During the first half hour, I kept questioning why a movie called 'Fullmetal Alchemist' features so little of the title character, and this persists throughout the entire movie: Ed and Al appear very little and don't actually seem to do much aside from infodumping and stating the obvious. I have seen numerous complaints that FMA 2003 was plagued by filler episodes, but even those offered insight into character motivations and backstories ergo they were of value to character development, if not to the overall plot. Since it offers no character development to any of the core cast of FMAB, nor any development to the FMAB story, this movie is, from a functional perspective, less useful than a filler episode.
Most of the action in SSM falls on the shoulders of Julia, the new main character, who, despite appearing more than the Elric brothers, is developed less than Rose from either TV series. Mustang, Riza and Armstrong appear solely to draw viewers in and have even less importance in the plot than the Elric brothers. The villains themselves hold little suspense if you're even slightly familiar with the FMA tendency of having a Big Bad behind the first villain.
Art - 4/10
And now we get to my second biggest complain about the movie: the art and animation. Bearing in mind that animation (movement and production) and art (character design and style) are two different things, it is possible for one to be better than the other, as is the case here, with the animation being notably better than the art. Contrary to the style we've become familiar with in previous FMA works, the character designs in SSM feature few sharp edges (making their hair look like putty — especially Ed and Julia's) and small pupils (which make them look permanently distressed), and their faces lack detail.
The action sequences have a noticeable sketchy and stretchy look, which fits as long as you don't linger on them. Animation-wise, the battle sequences and even general movement in SSM aren't as fluid as in the TV series, which was a disappointment considering the fact that, as a movie, SSM should have had more attention paid to these. Add to this the lack of closeup shots in favor of either zoom-ins or shots from far away (probably to mask the bad art), and you've got a very static-looking movie.
Sound - 4/10
The 2 points [edit: later bumped up to 4] I gave to this movie are mostly for the voice acting, which was on par with the TV series; I will blame the excessive shouting on the bad direction, not the actors. As for the soundtrack: while none of the OP or ED pieces caught my attention, the score was decent and supported the scenes well, albeit forgettable as the other parts of this movie.
Enjoyment - 3/10
Overall, I feel this should be avoided even as a fan of the FMA story; it adds nothing new and may even sour one's enjoyment of the previous stories in retrospect. Furthermore, most of it features mistakes in filmmaking in general: dumping too much information and plots at the same time, introducing many characters without giving them any depth, underusing core characters, and dropping what makes the FMA story interesting in favor of soap opera-level cliches — all of which make this movie a chore to watch.
Overall - 4/10
Personally, I only watched this for the sake of completion, but even as I was watching it I began to regret the wasted time. I'm normally a proponent of having people form their own opinion on something, but this time I feel I need to say this: avoid Sacred Star of Milos, you're not getting that time back.