For the Celtics, the only value is the pick, and better draft position in 2015. It'd be nice if they could get an unprotected first rounder from Dallas outside the Dirk window, like a 2018 pick to coincide with the Brooklyn haul, but that's highly unlikely. If it's Dallas' 2015 pick, this is mostly a tanking move and an attempt to get something rather than nothing. It means they've decided keeping Rondo as part of the next competitive Celtics team is not an option.
For Dallas, getting Rondo would be a fascinating experiment. They have a pick-and-roll point guard's dream roster. Notice how well Rondo works with Zeller on the pick-and-roll, because Zeller's great at rolling to the basket and has the hands to catch the ball on the move? Tyson Chandler is that times ten, with the ability to finish strong at the rim. Obviously, Dirk is the ultimate pick-and-pop big man, a talented screener who can bend defenses, and whose mere presence should allow Rondo to get into the paint at will. But at the same time, while Rondo is a great passer and decision maker on the pick-and-roll, his lack of shooting will close some of the space that would otherwise be there. I'll be interested to see if his passing is a net positive for Dallas on those plays versus a guard with shooting range like Nelson. These days, Dallas relies an awful lot on Monta Ellis, and he's been fantastic, reigning in most of the bad habits he'd had in Milwaukee and Golden State. While Ellis isn't a point guard, he is a lead guard that needs the ball in his hands to be effective. I'm not sure he has the skill set to be effective off the ball, and I'm not sure it's a great move for Dallas to pair him with another guard who's worthless without the ball in his hands.
That said, if there's a coach in the league that can find a way to incorporate what Rondo does well into a championship offense, it's Rick Carlisle. Dallas will be fun to watch if this trade goes down.