I'll just focus on the offensive side of things, since that's what most people seem to want to do.
As you can see, besides Geoffrion's small (impressive, sure, but small) sample sizes, Legwand leads the team in ES P/60 and G/60. He's also among the best when it comes to driving the goal-scoring in our favor (+/- ON ice minus +/- OFF ice).
Is it all secondary assists? Well, no, Legwand this year has more primary than secondary assists, if you feel that it matters.
So how is quality of linemates/competition factoring in? Not significantly, either way. He's roughly facing/playing with average players, respectively.
This is all Even Strength. Legwand hasn't performed particularly well on the PP, in the limited time he's received (seen here), though his shot totals suggest maybe he should get more work there. His PK work is also middling this year, so perhaps criticisms of his season should start with his special teams work.
The argument that he's overpaid is a little more subjective, so it's harder to say, but the fact is that Legwand (especially at even strength) is objectively probably our best forward (along with Erat) this year.
And, yes, I know nothing will be put to rest by this post.