I would also suggest reading the latest issue (2006-12) of the online magazine - reefkeeping (dot) com. There is an article on refractometers and how to make sure they are accurate (they can be just as whacked out of the box as a hydrometer), how to get them accurate and keep them there.
Summary of the article:
Refractometers are a quick and often accurate way to measure the salinity of reef aquarium water. Once checked to be sure that they were made correctly, they may provide years of service, providing they are not dropped onto a hard surface or into an aquarium. As with many devices, however, you sometimes get what you pay for, and sometimes less. Very inexpensive refractometers can be prone to errors and may need to be checked in a solution matching seawater, not just pure freshwater.
Other methods of salinity determination are also quite suitable for reef aquarists. These include conductivity using electronic meters, and specific gravity using floating glass hydrometers. Plastic swing arm hydrometers can be accurate, but seem to be more prone to inaccuracies than electronic meters and glass hydrometers. In general, it is good to calibrate any device used with a seawater standard at least once to confirm its proper operation before relying on it to gauge the salinity in a reef aquarium.