I own the model d stp and can give a few notes on it.
This stp works pretty good. I personally like the way their original one works better, but this one works better for daily packing. In order to wear it with a harness you would have to have a special harness which holds the balls back, like a home made one or like the one they sell.
The stp itself actually makes a decent packer unlike its predecessor. It needs some practice but once you practice some, it should work just fine.
I still far prefer the new peecock, but if youve got a lower budget, something liek this or the TSW STP should work just as well with some practice.