I have just recently added these to my leg day and I find they hit my hamstrings real good. I've been reading some articles on Dr. Squat's website and he describes SLDL as being a dangerous exercise if not done properly and he adds that you shouldn't go deeper than 2 inches below your knees.
"You must NOT go more than an inch or two below your knees. By the time you reach your (slightly unlocked) knees, your hip joints have fully flexed, and any further lowering of the bar is accomplished ONLY through eccentric hyperflexion of your spine -- a NO-NO!"
Here is the article itself:
How low do you guys go? Because I usually go deeper than that.