This card sent X-Men Leadership into the stratosphere. In combination with Utopia, 2-cost X-Men allies become Limitless Stamina with bodies attached. For Cyclops in particular, this is absolutely ridiculous, as he can pull every 2-cost X-Men ally available into his deck. (Playing Armor for a single resource to get a Hero-ready and two blocks is obscene.)
As powerful as that is, it's still not even the limit of what this card does. +1 HP makes all of your ally plays even more of a value proposition than they already were. Because the discount is not limited to once-per-turn or anything like that, playing Pixie into another ally in the same turn becomes astoundingly easy; a 5-card hand can get you Pixie and Gentle, who can put out 11 damage combined and defend twice. Using Cerebro to find the ally that fits your resource situation for the turn is even better when that ally's cost is reduced.
One thing to keep in mind is that, in order to receive the discount, all of your characters have to be X-Men, meaning that it only works in Hero form (barring Honorary X-Men). This also means that it doesn't combo well with Danger Room, which only works in Alter-Ego.
Another is that the value of this card goes up as you play more allies. It leans toward playing lots of inexpensive allies rather than a few expensive ones, and encourages cycling them rather than keeping them out in perpetuity; playing X-Mansion and the Training cards to extend your allies' lives will generally be less efficient than just playing another ally.
And lastly, because Uncanny X-Men makes it easy to play allies quickly, and those allies get extra HP, it can be easy to bump into the ally limit. (Playing Utopia heavily mitigates this issue; I really can't overstate how powerful the combination of these two cards is.)