This is kind of a sneaky question. "Successful" is an extremely subjective term. So to answer this properly, I'll need to define the term as I see it in this context. To me, a successful season would be one in which the team meets or exceeds expectations set before the season began. I'll even stretch this to allow for the consensus expectations, rather than just my own.
The consensus opinion, both around here and via the professional analysts, was that the Vikings were expected to improve, but not by all that much, remaining a sub-.500 team. By those lights, this was not only a successful season, but an outstandingly successful one.
Successful, however, does not equal satisfying for most folks. Some just have to complain, no matter what.