For me, getting a refractometer was one of those upgrades that made a lot of difference. With a refractometer you need a sample size of two drops. I figured that this would pay for itself in beer savings alone. I also had just broken my hydrometer and came to the realization that hydrometers are made of breakable glass that can break! 🙂
Refractometers can be used to measure gravity throughout the brewing process. You can also estimate the alcohol content of your beer if you forget to take an OG with the use of a refractometer and a hydrometer. Check out Sean Terrill’s excellent refractometer calculator.