I just ordered one. I talked to Trevor at 4sevens, and he said the deep carry pocket clip will fit this light. He said, the TurboX "should" be lego-able with the 18650 body, and "will" be lego-able with the rest of the quark line. (There have been issues with the 18650 body not lego-ing with other lights, which is why the 18650 body is being discontinued.) He also said the rechargeable Li-ion 17670 battery usually does not fit in the TurboX (said the battery needs to be no more than 16mm wide...the 17670 is usually 17mm wide. Strangely he said some 17670's will fit, but doesn't recommend buying one for this light because it would be a lottery whether or not the one you get actually fits in it.) So with this in mind I just bought a few rcr123 Li-ion rechargeables. I'll let you all know my impressions after I get it.
EDIT 6/13/11 - I bought two AW brand 17670 Li-ion batteries from 4sevens, and both fit in my Quark 123x2 Turbo X. A little snug, but still slides in & out without much effort. Works great too. Also, the deep carry pocket clip does fit no problem. I had to bend the tip of the clip out a little with a pair of pliers so that the light could easily slide onto my pocket.