I bought a refractometer last year and wished I had bought it a few years earlier.
I use it intensively and often compare it with my hydrometer readings.
The refractometer how ever can not be used -as is- for measuring a must that contains alcohol. There are conversion tables around for compensating that.
Mine is ATC that means that is compensates temperature.
So instead of filling a testing tube with must and waiting till it has the right temperature, I just use a drop of must and do not have to care about temperature anymore. Well worth the 50 euro's or so I paid for it.
If you have a decent digital cam with macro possibility you can take pictures right through the eyepiece of the refractometer which can give a nice extra to your winemaking log, just like here:
http://wijnmaker.blogspot.com/2009/02/basterdsuiker-brown-sugar.html
Luc