At it's core, the Justice League is an inherently bulletproof concept--the world's greatest superheroes (plus Aquaman) gathered together. It's curious then how often comic creators fail to "get it," whether by packing the team with Avengers-esque "single characters who don't work well outside of team books," packing the team with complete unknowns, or just generally having them spend issue after excruciating issue littering the page with interior monologues and bogging down the book with interpersonal soap opera.
Likewise, the Crime Syndicate is an equally bulletproof concept--they're the Justice League, only evil and on a planet that is also evil. Naturally, Comics are fairly happy to barely use them, as they've only had very notable appearances (in my lifetime, anyhow) at the beginning of Crisis on Infinite Earths (in which they died in an oddly powerful moment) JLA: Earth 2 (which formed the basis for the modern-day Crime Syndicate) and JLA/Avengers (which kinda echoed the beginning of Crisis, but wasn't quite as powerful, but was a clever roman a clef on the aforementioned scene) as I guess they don't fit into all the metatextual navel-gazing that they feel should naturally go on in a comic called Justice League.
Thankfully, as these predilections went into overdrive over the last decade, we had the DCAU, also known as "If you liked DC comic characters, but hated DC Comics, well, there's this." Funnily enough in the decade-plus run of the DCAU, they'd never done a Crime Syndicate story. the closest they'd come was the Justice League episode "A Better World," wherein the Justice Lords took over their parallel earth, Squadron Supreme style. It was a great pair of episodes, true, but it wasn't the the JLA vs. Crime Syndicate story you knew they could do.
The plan was, as I understand it, that when Justice League transitioned into Justice League Unlimited, there was supposed to be a direct to video movie bridging the two series which would, in fact, feature the Crime Syndicate fight. It didn't end up happening--at the time, at least.
The idea was filed away, however, and now it's time has come around again. However, it had been a few years, and Batman: Brave and the Bold had already done a pretty awesome two-part Crime Syndicate episode, so the question remained--was there room for two takes on the same concept?
Sure. While JUSTICE LEAGUE: CRISIS ON TWO EARTHS doesn't exactly hide it's origins as a Justice League finale/JLU continuation (seriously--it doesn't take a lot of finessing to see where this slots in, despite the different voice cast and animation style) it does exactly what it says on the box--you get the Crime Syndicate vs the Justice League, and a pretty decently-told story in between all the fights as well.
It all starts with Lex Luthor and the Joker of an alternate Earth (who are the good guys this time out) stealing the macguffin from a heavily guarded fortress and getting the evil versions of Hawkgirl and Martian Manhunter (who Edgar Rice Burroughs fans will quite enjoy) the Joker blows up those two and just in time for the main Crime Syndicate to show up, Lex vanishes to another Earth. Cue credits.
Things move pretty briskly from here on in--there's only 75 minutes in the movie, after all-- and the battles lines are swiftly drawn. The Crime Syndicate has risen to such power on Luthor's Earth that the governments of the world have basically taken a hands-off approach and lives in fear of the Syndicate. Luthor needs the League's muscle to take down the Crime Syndicate in such a way as to inspire the people of his earth to stand up for them--it's a pretty subtle but well thought-out idea that gives the notion that one ass-kicking isn't going to solve everything.
There's little clever touches all through the movie, actually--for one, the notion that the Crime Syndicate is actually a syndicate, wherein the five main baddies control legions of associated baddies (Superwoman leads an evil and suspiciously Marvelman-like Marvel Family, Owlman leads warped versions of the Outsiders) Deathstroke is the President of the United States, Ultraman, the leader of the Crime Syndicate sounds suspiciously like he's from North Jersey (not that I'm implying anything) and in an utterly throwaway recurring bit Wonder Woman gets her Invisible Jet.
None of these things, mercifully detract from the main point of the story--Luthor's discovery of parallel worlds has an unintended ripple effect. Owlman's built a bomb capable of destroying their Earth to use as leverage against world governments and finally take full control. However, with the discovery of travel to parallel Earths, Owlman has decided, as every decision one way or another is played out over an infinite number of universe, that the only decision that would matter, would be to find the Earth that started everything (Earth-Prime) detonate the bomb there and cause the whole thing to collapse.
So that's the basic plot in place, all that's left are the fights, and they're pretty tremendous. You don't want for action in any of them and they're clever enough twists in each of them (Batman vs. Superwoman and Green lantern vs. Halo come to mind) that it doesn't get monotonous.
In short, it's an effective Justice League story--with enough enduring elements to be classic, but with enough fresh spins on the familiar to be contemporary at one and the same time. Of course, this does raise the question of why comics, its native medium, can't seem to manage stories like this anymore.
Bottom line, y'all--for $20 at most, you get an awesome lead feature, the aforementioned "A Better World" and a handful of previews. If you lament the current mess that comics, and Justice League comics in particular have become--and you're certainly not alone there--well, here's something you'll really enjoy.