Here's another time waster for you. The prone hamstring curl. Just like all of the other time wasting machines, form is compromised due to the biomechanics of the machine. Again, all machines have their place in the world of fitness, it's just that they are best utilized in rehab programming (and really, this is not the best option entirely).
The big issue with the prone curl is the recruitment of other muscle groups, mainly the hips and back. While the hamstrings work in a cooperating fashion with these muscle groups, they should not be the primary movers in the exercise. You'll easily notice this compensation by watching the individuals (hopefully not yours) rear end pop up in the contraction. It's not really the most flattering position to be in.
Better alternatives involve the stability ball, of course. If you need to work on incredible amounts of strength, go for cable exercises. If you absolutely have to use a machine, use the seated curl rather than the prone. You'll be glad that your hamstrings will get a great workout and your won't be the one flashing the junk in your trunk for the rest of the gym to see.