I do not have a refractometer and am considering buying one. My own concern is how precise you can get the readings. How many brix are you plus or minus when you do a reading? Is it as precise as a hydrometer, or close to? Do you still use a hydrometer for accurate readings before and after fermentation?
Well a refractometer is very accurate for measuring OG. you do have to calibrate it using a wort of known gravity, but once that's done You are good to go with the refractometer. I am equally confident in my refractometer and hydrometer readings.
The refractometer is less useful for FG, due to the alcohol present in the solution. There are conversion calculations you can perform but generally speaking you still need a hydrometer for 100% accurate FG readings.
That being said I think a refractometer is a great tool for a homebrewer to have especially if you are all grain. A good refractometer allows you to check for conversion and check the gravity of hot wort without temperature correction and without having to pull a large sample out into a hydrometer test tube.
Refractometers are best used for pre-fermentation readings. I use mine to get pre and post boil gravities and then use a hydrometer to check on fermentation. The nice thing about using them on brew day is you only need a few drops which cools much faster than a full hydrometer sample. Many refractometers have automatic temperature correction, and hydrometers come with an adjustment chart for temperatures in a 10 or 20 degree (F) range - but the best accuracy is attained when your sample is at the temperature at which your instrument was calibrated.
You can use them on beer but need to apply a correction formula because alcohol throws off the reading. Here's an article on refractometer use and includes the formula for alcohol adjustment: http://www.brewersfriend.com/2013/04/24/using-your-refractometer-correctly-for-maximum-accuracy-in-home-brewing/