I'm a PT and i recomend people do these all the time. I have done them exclusively before, but I wonder about it because it is a static hold. Would a full sit up in the same circumstances with slightly more weight (to offset the 5 count) be more beneficial? I too did go down all the way after the static hold.
PS I just dont understand how the static hold is "great for deads and squats" Good yes, but better than a full rep? I dont know