For the Captials, I think this trade is also a good deal. Washington has had a great start to the season, and adding a veteran who has playoff and Stanley Cup experience can only be a good thing. Losing a third rounder hurts, but that draft isn't for another two years, so no issue there. The injuries to T.J. Oshie and Nicklas Backstrom have hurt some of the Caps' depth, so adding more will serve them well. Also, Eller is extremely familiar with the team, so getting him on board will be a simple task. Eller hasn't been a big goal scorer throughout his career, but he's a very good defender and a solid faceoff man, both things that any team vying for a playoff run would dream to have.
The Capitals may add even more pieces before the trade deadline, and I think that this trade symbolizes that the organization wants to not just make it to the playoffs, but make another deep run. If the Capitals can keep up the pace, they could be a popular destination for some of the bigger names near the deadline. I believe Eller is just the beginning of the reinforcements for Washington as they try for one more run with their aging core.