I just got an HP HBAR kit, and it has great build quality.
I'll break down the problems with it though.
1. Barrel lengths suggested seem to be wrong. It seems every length barrel besides very short ones will bounce around without spending some time with spacing (even 650mm). To make mine work I took an extra aug metal barrel jacket (the metal tube that holds the back part of the barrel in place inside of the outer barrel) and sawed it down, and filed it to fit perfectly with the flash hider, so now I essentially have a two piece barrel jacket that goes all of the way to the inside of the flash hider. I have heard of others being able to use 650mm and 590mm barrels without modification though, so it could just be that mine is weird.
2. Mine arrived with some light rust on it. I think it looks good like that though
3. The springs that hold the legs in place are held in by nothing but the legs themself, so if you pull the legs out by mistake (can easily happen while adjusting) the springs can be lost. But that is a simple fix as I just glued them to the adjustment button.
The overall build quality is excellent and impressively solid. It makes actually makes the gun about a pound heavier than the real AUG HBAR though (unloaded). If you get it, be prepared to do some work to make an inner barrel work if yours is like mine.