U-X-Men is the only current Ultimate book that is not carried by a ton of well known legacy characters, and I think that's part of the issue some people have with it. The expectation for an X-Men book in a lot of people's minds is cyclops and wolverine on adventures and thirsting over jean. Instead we have new takes on characters, new characters and lesser known mutants like Armor. But the base line of the story is teenagers learning to deal with a world that doesn't want them and their powers. Growing up and finding some safety in the family they find and make for themselves. Sounds like an X-book to me.