Just finished Justice League vs. the Fatal 5 now that it's on BluRay, and...yeah, this is a pretty good Justice League Unlimited movie. Hell, I don't know why they didn't just officially brand it a JLU movie, considering it's using the JLU character designs, sets, voice actors, and even the musical cues.
I've never been terribly interested in the Legion of Superheroes, so I found the Fatal 5 kind of generic & randomly powerful. My big issues with the film lie with the ending, though. Suffice it to say that one character pulls god-tier power RIGHT out of their ass when everything the film built up to that point indicated they SHOULD have been severely weakened. Also, there's a certain power source that plays right into the film's climax that the movie never addresses whether it can be replenished, and it's just...gone at the end of the movie, and the movie never gives a resolution to it. It's an annoying, dangling plot thread. I also would have preferred if the movie was longer, but that's just getting greedy.
One nitpick, though, I can't let slide is the animation. It's really inconsistent, reminding me oh so much of how bad the animation in Batman & Harley Quin was. Characters go off-model a LOT, particularly Wonder Woman (who looks like a bobble head in certain shots) and Superman. However, there's also a shot when the league is jetting to the finale where Batman's eyes go WAY off model. It's really distracting.
Overall, though, I quite liked it and it's a good return to form. Hopefully, they stick with the JLU designs and stories for a while. I know Hush is next (with Damien Wayne...ugh...), but it would be nice to see more animated films like this coming out of DC.