Similar to Deft Focus, X-Gene is an efficient resource upgrade that can't be built around because it only pays for cards that come in your hero's identify-specific card set. Among the mutants we have so far, here is how many identity-specific events they have that cost at least 1 resource and a comment on whether or not I would play it in their deck.
Wolverine (10) - Sometimes play. Despite having the most eligible events, there are two mitigating factors. 1 - Wolverine often spends all game in hero form, making this unplayable except on turn one. 2 - He can play one attack event per turn with the claws, so you can't use X-Gene as often as you like. Still such a high count of other events that it's reasonable to play the Gene.
Rogue (10) - Always play. Rogue needs all the help she can get setting up economy
Storm (9) - Always play. High count and powerful events. No-brainer
Phoenix (8) - Usually play. A bit hard to make use of early, but once you're unleashed, your events are your best cards.
Shadowcat (7) - Usually play
Gambit (7) - Usually play
Cyclops (6) - Usually play. Cyclops decks are often high-synergy and based around allies, so off-theme card slots are slim. That being said, Cyclops goes through his deck really fast, and Phoenix can grab an event back. I like it here.
Colossus (3) - Rarely play. It only pays for Steel Fist
Even with Colossus, X-Gene is never embarrassing to play because it takes only 2 uses to break even on resources, and you can often use it the same turn you play it. This card will be a staple of X-Men decks.