A couple of notes:
Overall I think the general consensus is that this card is not very cost effective in 1-2 player games. It is a good consideration in 3-4 player games
Cutupper is clearly a strong card, as suggested by the restriction of 1 per deck. It provides decent damage plus a stun status - something only identity-specific cards like Heroic Strike usually get to do. In 'Pool, you should automatically play this as far as I'm concerned, but I wanted to point out that this is an excellent Attack to bring in when playing Gamora. Barring any specific synergies found with other options, Cutupper is one of the best out-of-aspect attacks or thwarts to play on rate.
Great card... it practically fits into any deck, it's like "IronHeart" but better, since it activates when it comes into play instead of when it comes into play from your hand... Just remember to put a support that has the resource you can get the most to need ("Ipac" if you want mental "Utopia" if you want physical "DangerRoom if you want electric) If one day a support with universal comes out even better.
Some X-men allies, Rockslide included, don't seem to be that good for the cost. 4 is a very high cost to merely hit twice for 3 and go down against a villain attack or swing one last time. But you have to remember that a lot of the X-men allies are not great for the cost to begin with. The reason lies with the impressive amount of synergies they have with things like the Training upgrades and Danger Room. Insert Rockslide alongside these 2 and you suddenly have a 9 HP ally that can deal 12 damage over 4 turns AND take a villain hit before finally being discarded. Doesn't even need a med team for that value. There is no easy-ish way to do this with non-Xmen Protection allies with other traits.
Another great way to play Rockslide is with Mutant Protectors, in which case you cheat him on the board at 1 cost and defend with him. At 6 HP he can comfortably take a villain hit without getting removed most of the time with the added bonus of 1 Retaliate. You can then use him to attack or defend again the next turn, which can be quite good at 1 cost.
The target of the game is to knock out villain. This card gets insane value (1 ER per 5 damage) which is superior even to the hero specific events. However there is a small "but". First you need to reveal treachery. It can be something, that's automatically end your game. The harder scenario you play, the more dangerous this card is.
Summary: (A-C): B- in standard/easy scenarios C+ in rest.