Both teams got what they needed, but for New York that was a whole lot more than for Phoenix. Phoenix, with Amare, Zarko, and Leandro, are building towards the future. Stephon wasn't gonna be in his prime when all those guys hit theirs. Dealing him now and getting those first round picks was better than dealing him later and getting less.
For the Knicks, they really didn't give up much at all. We've seen them deal guys like Nene and countless draft picks in order to shore up for the present. The Knicks are supposed to be New York's varsity squad, and they plan to win now. So what they gave up didn't hurt them.
Getting Stephon is big. McDyess SUCKED. I can't put it any other way. And he was the biggest piece that they gave up. Isiah didn't like Eisley or Ward anyway, or Lampe for that matter, so those weren't big losses. So basically he gave up several undesirables for his perfect idea of a point, one that scores big and can pass big.
If this trade doesn't push the Knicks back into the playoff picture, I'm not sure what will. There are only a couple points(Baron, JKidd) in the East who can match up with Stephon. Almost everyone else will be made into mincemeat by his playing style. It happened in New Jersey before Stephon learned how to win. I think this is a highly different situation than the one which he left with the Nets.
Looking closely, it seems that Isiah is doing just what he did with the Pacers and Raps - try to style them after the Bad Boy Pistons. For that reason, I don't think Kurt Thomas will go anywhere soon, unless something impossible to turn down comes up, which I doubt. Marbury and Houston in the backcourt is huge, and Houston is by far the best shooting guard that Stephon has ever played with. That tandem can knock most teams apart, and when Van Horn and Houston's shots aren't falling, Stephon can penetrate as only he can do so well.
I think Van Horn, a la New Jersey, will get isolated from the flow of the game at times, but that with the presence of Houston, it won't matter so much(to the Knicks). It might anger Keith, but if he really wants to win he'll accept the diminished role quietly and perform it to his utmost.
Overall, this sets the Suns out of the playoff picture not just this year but for several years, and puts the Knicks in the thick of the playoff hunt.