I have had one for a year and a bit.
I use this site:
http://onebeer.net/refractometer.shtml, to convert my og brix reading to gravity. (software like felten is saying)
It agrees with my hydrometer og.
My hydrometer does not agree with the refractometers in-bulit scale for gravity, so i trust the brix only on the refract.
Using the above website for fg readings, the corrected brix reading is always 3 to 4 points lower on the refract, so its not accurate/precise for fg, but using my refract the fg of my beers always seems to get to about 5 brix. If i mash higher they will finish a little higher.
I find the refract a useful tool to have. 30 bucks is a good investment if you are planning to measure the gravity of wort regularly.