I agree with the posts above, that they are great for procedures and that certain manouvers - rejects being mentioned already, are well done. As it goes for training in Transport Category aircraft, there is no subsitute because the real thing is just too dangerous.
However, I find that it is the little cues that I miss in the sim that throw me off balance. For example in the Dash 8 you are sitting right above the nose wheel, and it gives you this little bump when you extend or retract it that the simulator does not reproduce, (same with the PTU) or the actual feeling that you get with the nose up moment when you extend the flaps - it visually reproduces it, but as someone already stated, with a small, but noticeable, delay. These events, whether a pilot realizes it or not, very quickly become cues to a part of a checklist that you do not realize when flying the line, but when you dont get them in the sim youre sometimes sitting there wondering what to do next - especially if the training is not going well!
As for turbulence, its more like sitting on your chair at work and bouncing up and down - not really realistic at all, although for some reason sim trainers always have it turned on!
To describe the sim sickness that someone mentioned earlier. My last ride in the sim (just last weekend) was the first time in almost 2 years that I have not been disorientated in the sim. For me it was always the rapid repositions following a sequence - I never threw up like some guys have, but it sometimes came very close (especially if you had the eggs beny and beans at the hotel in the morning!)