A really good Apocalypse's story
This story has a lot of fights and drama, this tale is why I began to read X-men, the plot is similar to the cartoon serie's episode "one man's worth", but more interesting, the art is really good, and the pages has a lot of color for an old reprint, I think it is a must read.

One of the most important in X-Men history
A while ago I went through and read X-Men (vol. 2) #1-113 & Uncanny X-Men #300-400 simultaneously. After the Age of Apocalypse that came out around 1995, the X-Men titles went downhill fast. For two straight years (about 25 issues), the X-titles were just plain crappy -- mediocre at best.
And then this story came along.
This was the best, most intense storyline the X-Men editors/writers cooked up in quite a while. And it was incredibly significant, because it finally wrapped up the storyline that'd been brewing for 12-15 years about the villain Apocalypse. This storyline was extremely significant to many characters in the mutant community; for some, like Cable, this villain defined them over the years. So, Apocalypse finally puts his master plan -- the thing he's been working towards for 100s of years -- in motion.
This storyline is just as significant as Mutant Massacre, Fatal Attractions, or Days of Future Past. It's infuriating that recent writers have cheapened it by bringing Apocalypse back in '07, thereby breaking the last straw for my fandom. This storyline is also significant because it was the first of many stories where the quality of writing kept improving toward greatness once again, leading to Dream's End and, of course, Grant Morrison's run.
I haven't read this particular tpb. But if it's collected in the order of the issues as they were released, then it should read just fine. I didn't have any problem reading through the individual issues. Yes, Ages of Apocalypse & (the hopefully included) Search for Cyclops are also essential to this story. It may be possible that the Astonishing X-Men (vol. 2) tpb may still be necessary, as it introduces Apocalypse's new Death character, which reveals Wolverine to be that character and thereby leads straight into The Twelve story arc. But, I don't know yet how they organized this trade and that 4-issue prologue may not be necessary as part of it may be included here.
Oh, and get it at a discount price, if you can.[...]is pushing it. The Age of Apocalypse story was the only softcover worthy of that price. It's doubtful this is that hefty.

poor read
This is almost waste of money. They didn't include the whole death storyline/ It begins right after Death is revealed to be wolverine. The Artwork is inconsistant. On one page X-force is in street clothes and in the nest they are in uniform. Did they all take a break in the middle of the fight to change clothe's. In order for this storyline to somewhat make sense you have to have the Ages of Apocoalypse(not Age of Apocalypse) and the search for cyclops and if Marvel never releases those then this book seriously worth skipping because it is just going to confuse the hell out of you. also $30 is way way too expesive for a book this size especially when they leave out so much of the story. its about 1/4th the size of the Age of Apocalypse trades.

Just plain awful
Before Grant Morrison re-vamped the X-Men with a refreshing take on the mutant mythos, there were some pretty bad X-Men stories that were out there. The Twelve is one of them (and also one of the reasons I myself tuned out of mainstream comics for a long time) as Apocalypse sets forth another one of his "master plans" that involves the twelve most powerful mutants on the planet, like Professor Xavier, Cyclops, Phoenix, Cable, and the like. What makes The Twelve so bad, besides the fact that the entire story is an incoherent mess, is that little to no sense is really made of the proceedings. The twists that occur, such as Death being revealed as Wolverine and the "death" of Cyclops, only add to all of this, and that's just for starters. Horrible dialogue, uneven sequences, and artwork that represents the worst of the mainstream superhero genre (hello Rob Liefeld!) only contribute here as well. All in all, no matter how die hard of an X-Men fan you may be, The Twelve is best left on the shelf no matter what. Too bad Marvel just couldn't leave this storyarc in the past where it belongs.