I usually start off with a pretty light set of hamstring curls and leg extensions to help warm up for heavy squats. Don't worry though, I do them with a towel over my head and the shame that should be felt when going anywhere near the "machines". You know what, I'm going to quit doing them now. I just can't take the guilt now that I've admitted it in public.
I will say I have been doing them because of rehab (I think single leg SLDL is working better) but to honest other than that they are garbage. Machines DO NOT make people strong. Muti-joint compound exercises are what makes people strong. GHR, Good mornings, and SLDL will do a far better job of bringing strength to the hamstrings, glutes, etc.
Others feel they're a useful exercise to have in the regular arsenal:
They may have value as a rehab exercise and a warm up but generally I don't like 'em.
After i do all my squat exercises and other movements I do leg curls and leg exts. to keep the area around my knees strong.
The thread is at Monster Muscle.
Leg curls have always been and always will be a staple exercise for me. They work incredibly well, and there are several variations that can make them incredibly brutal.