OK I will go against the flow. I think it is another toy for the boys. But each to his own.
My son has one. I never use it because it is a little difficult to use compared to a cleaned after use hydrometer. If you don't clean it after use the refractometer will be off and if you don't clean the water test kits your tests will be off too.
I couldn't go through the picky calibration required by a refractometer and go to the trouble of buying the calibration solution before it gets stale and all that stuff.
I wonder how many reefers who own them go through the calibration stuff or simply just use it?
It is easier to simply plunge the hydrometer in the tank and scoop up the water to be tested and viola, you are bang on again. (and clean it after use)
Take a look at the this thread in advanced acquarist. It seems to confirm what I have posted.
http://www.advancedaquarist.com/2006...=refractometer