It's not a bad movie -- the action is good and the animation is excellent -- but, while it's great to see Ed and Al in action again, "The Sacred Star of Milos" is a bit of a let down compared to the series itself (either version).
Were a newcomer to watch this as a standalone movie in the Fullmetal Alchemist universe they'd probably enjoy it more than existing fans who are repeatedly forced to wonder what happened to the laws of alchemy and the entire point of philosopher stones. On the other hand, a newcomer would have a lot of questions about everything else as this movie was clearly made with only Fullmetal Alchemist fans in mind.
The director explains in his commentary that this is a different land so the alchemic laws are different (though Ed and Al have travelled just as far if not farther before, albeit not in this direction) but this isn't mentioned in the movie, doesn't explain the antagonist's overwhelming powers while he's in Central City or why Ed and Al can't catch him (or anyone for that matter), puts into question the entire point of the series if all Ed and Al had to do was travel Southwest to Creta or beyond where the laws of alchemy are supposedly different, and doesn't explain the unnecessary rebound after the disappointing climax. Fans will also catch on to the truth about Table City long before Ed and Al who've encountered the exact same thing before.
Overall, this movie is worth watching, but Fullmetal Alchemist fans should be aware this technically canon movie does a terrible job of following canon.