It'll be interesting to see what the Jets gave up to get Marshall. I think this could be a really good move for them. They have the cap space to pay a few big FAs, and Chicago wanted to get rid of him badly enough that he shouldn't cost too much.
The Jets still need to figure out what they're going to do at QB and sign some OL help, but it's not like they're hurting for cap space. This doesn't really preclude them from doing anything. Shame for the Jets that this draft class is so shallow at QB, because in a lot of ways they're a QB away from having a pretty solid offense. I assume they're cutting Harvin in order to make the Marshall trade a net positive against the cap, but they can afford to keep him and have a Harvin/Marshall/Decker/Amaro receiving corps. That's one of the best in the league, although it can only do so much for you if your OL sucks.
I don't think you can afford to make too many of these trades--trading away picks for high-priced veterans--without sacrificing your cap integrity, but in the Jets' case Idzik was so effective in cleaning up their cap that they can afford it. This makes it more urgent that they hit on what draft picks they do still have, but it's definitely easier to envision a scenario where they're competitive than it was a few days ago. If I was a Jets fan, I would recognize this trade as something of a risk, since there's some real opportunity cost attached if you get Miami Marshall rather than playing-with-Cutler Marshall. But it's a risk worth taking IMO.