So far, I've only used the Comp-Tac Shirt Tucker... it was GREAT on making the P40 I had disappear... Very comfortable too. The only thing I didn't like was the single "offset" belt clip. On the good side, it made a flatter profile than a clip on the holster, but on the less-than-good side, it allowed the cant to shift.
No longer have either, but that's a different story.
I just ordered a FIST Inc. "Universal #1" Kydex holster with IWB straps for my latest (and 3rd) K40. I'm very anxious to get it in and try it out.