I haven't done it, but on previewing, it's my least favorite of the series. The stretches aren't really held long enough for improving flexibility, IMO. The music is horrible (again IMO) and barely audible. It isn't really broken down into body parts. There are three different 20-minute stretches: one with no equipment,one with a stability ball, and one with the band (which I would use a NON-stretchy band for, since the object is to stretch our muscles and not the bands!)
I will try it, but have a feeling I won't keep it for long! (Another thing that I don't like about it is that Cathe doesn't talk much through it, and I like to have her talk me through things).
As for stretching in general, I think it's important to stretch after every workout, and to have a specific stretch (or yoga) workout once a week (or even two). P90X Stretch X is very good. About 5O minutes long, chaptered so you could choose just certain body parts, and it group certain stretches (shoulders, lower body, etc.)