The study is historical -- it assumes that tomorrow's lawyers will fare as well as yesterday's lawyers -- and the study stops in 2008, before the recent downturn. I don't know what will happen in the future. The price of a JD is dropping rapidly for this cycle and presumably the next cycle as well, as schools massively discount their prices. That can't continue forever, obviously. And the sticker price keeps going up and it's not clear if the increasingly porous nature of the profession will reduce long-term returns as compared to the returns my generation of law grads enjoyed. In short, the study is be a useful look back at history, but there are lots of questions about its usefulness for the future.
The study also doesn't do a detailed accounting of the costs of the law degree. Note, for example, that many of the schools with the very worst employment outcomes are the schools with the most debt-laden students.
UPDATE: As noted in the comments below, the authors of the paper will have a five-part series at the Concurring Opinions blog. Part one is here. Critiques of the study can be found here and here. Paul Caron has a round-up here.
[edited since posting]