Supposedly, the refractometer sold by Filterguys is the only one out that's actually designed to measure seawater.
The rest seem to be designed to measure table salt (NaCl) solution, which means you can't calibrate them to zero with distilled (or RO/DI) water and expect to get the right reading for your tank water, because seawater and table salt solutions have different indices of refraction.
Randy Holmes-Farley wrote an informative article in the December 2006 issue of Reef Keeping magazine.
The other option is to get a standard (cheaper) refractometer and calibrate it to 35 ppt following that article.
I'm pretty certain it's the same one I have, which is the Captive Purity labeled RHS-10ATC.
If you read down on the Premium Aquatics website it says that you should buy the Pinpoint (American Marine) salinity calibration solution and calibrate the refractometer to 35 ppt, as stated in Randy's article. As long as you do that, my understanding is this will be a perfectly good refractometer and will last you forever as long as you don't drop it or something.