When the Braves acquired Derrek Lee on August 18th, most people considered it to be an act of desperation for a team that was fading in the standings and whose first baseman was in need of a DL trip. Lee did not seem like much of a solution. Sure, he had a good track record, but his 2010 season had been a bust. He hit only .251 / .335 / .416 for the Cubs and was bothered by injuries to his thumb and back.
Indeed, Lee's Braves career did not exactly get off to an auspicious start. He was already hurt at the time of the trade, and in his first 8 games hit just .138 / .242 / .241. In other words, it felt like Troy Glaus was still playing first base. Though the Braves went only 3-5 in this stretch, they maintained their division lead.
Lee turned his season around starting August 30th. From then until the end of the season, he would hit .330 / .424 / .530, serving as basically the Braves' only consistent offensive threat during September. Sure, the Braves lost their division lead in that time (and nearly the Wild Card, too), but you can't blame that on Lee.
It is not at all a stretch to say that trading for Derrek Lee made the difference between the Braves making the playoffs and missing them. If you like WAR, both FanGraphs and Baseball-Reference agree that Lee was worth 0.8 WAR during his tenure in Atlanta. Since the Braves made the playoffs by the slimmest of margins, that 0.8 WAR likely turned out to be crucial.
If you don't like WAR, let me just point you to Lee's signature Braves moment: on September 19th against the Mets, Lee hit a grand slam in the 7th inning to break a 2-2 tie. The Braves went on to win 6-3 (Lee also doubled and scored the Braves' second run). Without Lee, the Braves most likely lose that game, and quite possibly miss the playoffs.
In the playoffs, Lee's bat (like most of the other Braves') went quiet. He managed only 2 singles and a walk in 17 PAs against the fierce San Francisco pitching staff.
Lee is now a free agent, and with Freddie Freeman waiting in the wings, Lee's time with the Braves is almost certainly over (though I would not be against re-signing him to a 1-year deal). Despite his playoff flameout, Lee did about all that could be expected of him: hit well, play good defense, and help the team make the playoffs. He did it all while playing hurt, too.
Whether that is worth what we gave up for him (Robinson Lopez and two other prospects) is up for debate, of course--and that debate won't really be settled for years to come. Regardless of how those prospects turn out, we should remember that Lee helped give Bobby Cox the ending he deserved: one last playoff berth. I'm not sure that anyone can put a price on that.