Good story, not-so-good art
Wow, it's Giant Sized X-Men #1. When I read this the first time, I enjoyed it every bit as much as I still do today. I found the plot exciting, the new characters (especially that legend Nightcrawler) creative, but there's one thing about this issue I don't like: the artwork. Sure, Dave Cockrum is really notable in doing the art for this legendary issue, but that doesn't mean his art is that good.
Apart from the art, this is a really good issue. The thing that makes me enjoy it so much is the action. Even in the scenes without any real action, such as Prof X talking to Sunfire, you can still get the sense that they need to hurry before the X-Men are killed. It has the return of some old characters like Banshee and Sunfire, and some legendary new ones. This is the kind of issue that only comes out once every three or so years. It's brilliant.
But the best thing about it is the fact that they're all fighting a giant island. Didn't see that coming.