I began working in the field when I first started. It was definately good experience, but I agree that you shouldn't be slumping concrete and testin' density for too long. It can be, as others have said, a challenge to win back your position, with your boss and peers, as an engineer rather than a field tech.
During my field time, I had to tell many a contractor that they were not getting density, or the concrete was out of spec, etc. Interestingly, I found that it was frightening to confront the contractor at first, but it eventually got much easier with time. There have been a few times, under certain conditions and with certain contractors, that it was actually enjoyable...