So as someone who's been very frustrated by Stafford's play this season, let me give a brief description:
First of all, he's not good defensively at all and wasn't even when he was producing. He's really clueless in the defensive zone and has poor defensive effort levels. This is even more of a weakness this season.
Second, he's not been very good offensively at all, despite getting a lot of PP time and quite a bit of ice time in the top 6 of Jets, either. The play seems to die on his stick more often than not. However, at his best he was a functional, solid scorer although he was really streaky. Basically, as a player he's negative impact when you remove his scoring.
If he does regain his decent ~20+20 form he's a valuable top 6 guy but if not, he's a liability.
Of course, the cost was very minor and it is possible that a player like Bergeron might compensate for his complete inability to play defense. Hopefully he gets it going. He doesn't seem like a bad person.
And certainly, if he was brought in as a Jimmy Hayes replacement, he likely is much better even if he doesn't improve / regain his form at all.