Personally, I touch and go with SLDL. I have done them both ways and they both have merits, but for me right now -- I'm using them as a DL GPP exercise, so I just need general specificity.
I also have to confess that my form for SLDL is terrible. It looks like a regular DL without the hip-drop at the beginning. That said, I'm not sure it's entirely bad, so I'm still debating whether or not I want to fix it.
I feel like they are just a GPP exercise. It will increase the potential of your other exercises, but *by itself*, I don't think it will make a huge impact. In the right exercise grouping, it's fantastic.