i had hubcentric bolt ons and had all sorts of problems. :/ The nuts weren't sitting flush w/ the spacer (barely - couldn't tell just by looking at it though) and caused the wheel to not seat properly which in turn caused major vibration issues and issues w/ the lugs coming lose. Everybody kept assuming it was the stock studs that were too long but even after shortening them to where you could visually see that they weren't touching the wheels, there was still vibration. After inspecting everything and realizing what the problem was, I ordered H&R's w/ longer studs and haven't looked back. I had to ruin some tires and coilovers and go through 5 spacer bolt keys before i figured it out unfortunately.
I had the same brand bolt on spacers but in 20mm on the s13 and never had any vibration issues. I think that was b/c the nuts easily sat below the face of the spacer.
I would say if you're going over 15mm, get bolt ons - especially if the car's seeing track duty. Under 15mm, get slip ons. That's my opinion and real world experiences anyway.
