I agree with much of what was said above. Their projections are definitely lower than others. I have a spreadsheet with projections for each player from all of these sources:
Bill James Handbook
Ron Shandler's Forecaster
and PECOTA's projections are the lowest, generally, especially in forecasting playing time. That said, the Fantasy Baseball portion of baseballprospectus.com ups the playing time projections.
PECOTA was originally designed specifically for pitching and I think it does the best job out there for pitchers.
As for the book, if you love baseball, it's a must read. For fantasy baseball purposes, they do have a paragraph or so about each player that can sometimes contradict the PECOTA projection. So, where PECOTA might see Oliver Perez as having an ERA well over 4.00 this year, the comment on him is much more optimistic.
Roger Angell: I was talking with Bob Gibson and I said: 'Are you always this competitive?' He said: 'Oh, I think so. I got a three-year old daughter, and I've played about 500 games of tic-tac-toe with her and she hasn't beat me yet.'