I use the Hanna ULR phosphorous. It measures phosphorous in ppb. It detects at lower levels than the Hanna ULR phosphate detector. I have an SPS dominant system so I keep nutrients pretty low.
That said, I'm not managing any different than I did with Salifert testing. .01-.05 really doesn't require any different actions on my part. When it gets to .08-1.0 sure, but I could easily see that with cheaper kit. I'm even thinking I may try API.