The T-15 is a nice lower. Finish is excellent, it's deep black. I have a few (maybe four if I look hard enough) and the ones I have built have not have any problems with LPK's. I use RRA. They do have the take down capture screw already installed along with a tension screw just in case you need/want to eliminate play. I've never used it. They have very limited markings and the left mag well is clean, no roll mark at all.
Some people had problems with the fit of the safety. I didn't and don't remember the exact problem, but I assume it was addressed. Also, you may read that the FCG pin holes are too big. They are drilled at .156 which is what the blueprint calls for. They use standard .154 pins. I have one with .155 pins and an RRA two stage trigger and those pins fit fine, too.
These lowers are the same ones Denny/Global Tactical uses for his lowers with his name on them.
I just looked at Rainer's price, for that you can't go wrong. But, there are other lowers for about the same price or less.