I'm very happy with my Hagen's PO4 kit. HOWEVER, if you buy one open the box and make sure the #3 re-agent bottle is glass. If #3 is plastic the whole kit is worthless, and you'll get 0 readings. Something in the plastic reacts negatively with the solution and renders it useless.
I had to e-mail Hagen's and they sent me a #3 in a glass bottle(no charge) and it works very well.
Len
Edit: Are you workin' on that site Starry?