I truly do think that when you are in well-established labour, the water breaking will speed things up. With my first DD, my waters broke at home after about 2 hours of contractions. She was born 4.5 hours later. (not too bad for a first baby!) With my second, I laboured for 6 hours before my ob offered to break my water, if I wanted. I said sure! DD2 was born about 45 minutes later. With my third, I laboured for about 10 hours without "progressing" (stuck at 4cm). At about 6pm (I think!) my midwife agreed to break my water (I asked), and things became MUCH more intense from that point on. DD3 was born about 1.5 hours later.
In my experience, it definitely speeds things up. Whether it's a good idea or not is an entirely different question.