Some of the talk in the Thunderbird III thread got me thinking about Sage. She (along with Emma) is one of the few new members of the X-Men who has stuck in recent years. She may not be with the team currently, but she served with them for some time and there isn't a sense that she's likely to disappear into limbo and never be seen prominently again, as there is with characters like Lifeguard and Cecilia Reyes. She feels, I think, like a real X-Man.
So, it can be said that Sage is a success and, on the surface, you could argue that this is because she already had a history in the X-world as Tessa, Xavier's spy in the Hellfire Club. However, she was by no means a major character before joining the team as Sage and received little serious character development as Tessa. It strikes me that Sage works as an X-Man because she has a strong personality, unusual powers, and has developed interesting relationships with other X-Men that are unconnected to her past and, I think, that's the reason she's a success as a character.
So, do you think I'm right? And therefore does that mean that new characters can be introduced into the X-Men successfully and that the Thunderbird III/Lifeguard/Slipstream/Maggot etc pattern of new characters failing is just coincidence? Or do you disagree? Would we have been accepting of Sage if we didn't already know her as Tessa?