I rebuilt my stock PS box myself. My box does work, but it's not right. It makes some strange noises, doesn't respond quite right, etc. If I could do it again, I wouldn't even attempt it. In fact, I'm either going to send it out to have it rebuilt by a pro or will just buy a new one as I want my steering to be perfect. It isn't worth dying over a few hundred bucks.
Rebuilding that box was probably the hardest thing I've ever done, mechanically anyway. It requires a couple of special tools and the ball bearings are an absolute nightmare to get in place correctly. I'm not even 100% sure they're in correctly now. For all I know, I could have a ball bearing bouncing around inside of the box destroying it like somebody mentioned earlier in this thread. Getting the teflon seals in place is also another adventure...took me (2) rebuild kits to finally get them on and get the piston in without messing them up.