The majority of data available indicates that pre-training stretching neither reduces the frequency of injury nor effectively improves flexibility, the two areas in which it is supposed to provide benefit. Studies of marathon participants failed to show a difference in injury rates between athletes who stretched before the race and those who did not. But wait, there's more! Evidence from studies on vertical jump and broad jump performance indicates that pre-event stretching actually reduces power output, and other studies suggest that this is true for other explosive activities, such as weightlifting, as well. This may be due to a reduction in the effectiveness of the stretch-reflex portion of the concentric contraction caused by proprioceptive reset during the stretch.
If pre-training stretching doesn't increase flexibility or reduce injuries, what does? Proper warm-up safely does both. The loaded human body moving through its maximum range of motion actually provides a stretching stimulus for the antagonist muscle groups, the very ones that are tight. (The agonists cause the motion around the joints, while the antagonists resist or decelerate that motion. A lack of extensibility in the antagonists is the usual cause of flexibility problems.) A number of studies have shown an increase in flexibility as a result of complete-range-of-motion weight training. Improvements in hip and knee flexibility on the order of 40% or better are commonly experienced. Thsi is because proper form requires complete range of motion of the involved joints and, if proper position is maintained, the weight puts the body into a properly stretched position at the bottom of each rep, exposing the antagonists to a stretch stimulus each time the load is moved. This obviously requires good form, and good coaching. Properly done, each weighted rep provides a better stretch than an unweighted traditional stretch, because the complete range of motion is easier to reach with the help of the weight. More importantly, and most especially for the hamstrings, the postural position of the back--the very critical lumbar extension that must be maintained to fully stretch the hamstrings--is best accomplished with a loaded spine, since the load gives the spinal erectors some resistance to contract against for efficient curvature. It is common to see athletes attempt to stretch the hamstrings with a rounded lower back; this cannot be done effectively.
If traditional stretching exercises are desired, they should be done at the end of the workout, when the muscles are warm and the stretch will not interfere with performance.
Thinking about buying one here soon to help with conditioning workouts.