Derrick Williams is an athletic 4. He cant play the 3 because he has a lot of the same problems Josh Smith has, in terms of not being quick enough to guard 3's, nor be able to score efficiently against them on the other end. Against 4's though he has the quickness advantage and can better get his own shot.
The problem here is that SAC has 4 PF's now, and Salmons/Outlaw at SF. They must have another trade in the works involving Thompson and or Patterson otherwise this deal is just another terrible offering from a very mismanaged team.
DWill could end up being a very nice fit at the starting 4 besides Cousins, but again, they need to get rid of the glut of trash they have on their bench.
MIN got a good deal out of it with the Fresh Prince. Him and Love were teammates in college, he can defend 1-4, and he's just a great glue guy for a winning team. They needed more depth at the wing, and he'll give them that, plus he can defend bigger SF's who Brewer generally struggles against.