I like Derrek Lee. Why?
He's not THAT old. Sure he's an old 35 (will turn 36 before the end of the season next year) but it's only 35. Trusting a player at 35 & 36 doesn't seem like a terrible idea. Eventually he'll fall off the cliff but if you don't think that was last year (and I don't) than it's fine to gamble on the short term.
He bounced back decently with the Braves. Over the last 39 games his line was .287 / .384 / .465. That seems pretty much in line with what I'd like to see from a 35 year old Lee.
He wasn't that bad with the Cubs
April : .205 / .327 / .352 (.222 BABIP)
Post -April : .267 / .337 / .433 (.311 BABIP)
That line for BABIP (batting average for balls in play) is usually pretty stable. For Lee (for anyone really) .222 is terribly low. It's usually is an indication of unluckiness more than a true slump. .311 is close to what Lee would usually have - so it's not like he bounced back with a run of luckiness at that point. Would the Nats be happy with .267 / .337 / .433? Probably not. Would they be happy with something between that and the Atlanta line? I think so.
I don't see much bad in the fancy stats to indicate trouble to come. His line drive percentage is pretty stable, walks are stable, K's are up but not past some of his past season lines. The FB% and HR/FB dropped a bunch but both were oddly high in 2009. That was the aberration and it makes last year look worse. In truth, 2010 is closer to career averages. He is swinging more at pitches outside the strike zone (18.7% -> 20.8% -> 21.5% -> 23.1%) but as he does that he gets better at making contact at these pitches. (52.5% -> 57.4% -> 60.5% -> 65.6%), so that mitigates the problem a bit. It's not ideal, I admit, but it shows an ability to adapt.
His defense has been consistently decent. He's not the best but it seems like whatever efforts he has made in the latter half of his career to turn his fielding into a plus is continuing to work.
So I like Lee, for a year or two (next year's first baseman market is dry unless you want to give Prince Fielder 150 million). Maybe see if Marrero is ready or if Bryce is moved. Three years? No way.