It's definitely a good trade for Minnesota. Cassell can help Hudson at the point and also play shooting guard. He's a huge upgrade over Peeler. It's also insurance if Terrell Brandon can't come back.
The Bucks get nothing out of this except some salary cap relief, since they'll cut Peeler before the season, and a few minutes from Joe Smith in a bench role at power forward.
If the Bucks don't sign Gary Payton, all they would have gotten out of their big three - Big Dog, Ray Allen and Cassell - was Desmond Mason, Toni Kukuc, T.J. Ford and Smith. That's far too little.