A very specific card. You can use it when you are in AE and other players in AE may use it if they are a Mutant (According to Hall of Heroes). This is the greatest point of confusion with this card. The only real negative point about Danger Room is that you need to be in Alter Ego making it hard to use in true solo in some cases.
Now to the positives. When used in a X-Men Voltron deck, it's good. Grabbing a training card (which by the way are great!) is very strong as they give a massive boost to the ally. Even if you don't want to Voltron one ally but have many good ones out it supports you quite well. In a multiplayer game where everyone plays a Mutant, everyone can use it, making it even stronger. It being a basic card just rounds up its overall usefulness!
All in all, I would call the card mandatory if you play a deck that focuses on getting out X-Men allys or even more in a multiplayer game with multiple X-Men decks.
Edit: changed because of Hall of Heroes ruling.