Hard to answer this without knowing your format or your team's needs as Ellis and Howard are two very different players.
Ellis' value looks set to spike with the arrival of Reddick. I first thought that this trade might squeeze him a little, but now he is actually getting more minutes as he is backing up Jennings at the 1.
If you have excess bigs and this deal helps balance your roster, then it is fair. Howard's injury risk is not going away anytime soon, but I would not make this deal unless you have other strong C options and need guardplay.
