Came across this interview over at mixonline.com, wherein they talk to three veteran production sound mixers from film and TV: Jeff Wexler, Mark Ulano and Glenn Berkovitz. The article isn't exactly new, but hey, I never promised you a rose garden.
...At best, their work is utterly transparent, not calling attention to itself. At worst, well, if you have to struggle and strain to understand what the characters in a film are saying, or noisy backgrounds overwhelm the dialog, or the finished film is overloaded with badly done ADR, chances are that the production sound mixer had a rough outing. And it probably wasn't his or her fault.
Link to the interview. Also, be sure to drop by Jeff Wexler's website.