This collects the original run of the X-Men from issues 11 through 21. Number 11 was the last Stan Lee/Jack Kirby collaboration, introducing yet another new character (as they did in most of the first ten issues), the Stranger, and ending the Brotherhood of Evil Mutants storyline that had lurked in the background of the book for a long time.
Number 12, with Toth illustrating over Kirby lay-outs, is among the best comic books of its time, a tense tale of danger from another classic new villain, the Juggernaut, who gets polished off in number 13 (drawn by Roth over Kirby layouts) with a guest appearance by the Human Torch. 14-16 introduce the Sentinels, a deadly metaphor of mindless prejudice that wouldn't get built further for many years - the story isn't actually as good as I remembered it, though there are some great moments in that first part.
Number 17 tries to recreate the tension of number 12, but doesn't quite get there, even though it neatly sets up the return of Magneto whose ridiculous scheme is too easily defeated in number 18. 19 introduces the Mimic, and the limitations of Roth as an artist, no longer powered by Kirby layouts, become obvious - he's good at civilian clothes scenes, but kinda static in action. 20 & 21 are a two-parter by new writer Roy Thomas, who uses a lot more words and possibly less logic to a battle with Lucifer - the pages wherein the individual X-Men are on different levels of the mega-computer used by the villain do look pretty cool, though.