There are some interesting comments about Field in this interview with Arnold Steinberg, a Republican pollster/strategist that is a straight-talker.
http://www.flashreport.org/special-reports0b.php?faID=2005110202424596
Big deal, they are good at the final poll in major races. Since there are rarely major upsets in the final weeks, so is anyone who reads the news.
The Field poll has some major, major flaws, starting with their sampling. They sample counties based on population not on the number of people in those counties that actually ever vote. So San Francisco, for example, is always oversampled. It's why things always look bad for Republicans and conservatives early on in the Field poll, and then get better later.
The really good private pollsters can call successfully races much farther out than Field does.