The one Fruitnut linked to is a good cheap refractometer for most growers needs and I can relate on how it has enabled me to grow sweeter fruit with less watering. Without the refractometer, I had to guess how sweet my fruit are but now I’m able to give objective numbers to verify a change in my gardening routine did in fact lead to sweeter fruit.
If you grow very sweet fruit, you may want to get a 0-32 Brix and a 30-60 Brix meter. Some fruit like certain varieties of mango, Cherimoya, Sapodilla and others can have Brix ranges out of the smaller range meters.
If you attend fruit tastings, digital refractometer make it easier for everyone to verify the Brix readings. I have thus Hanna HI 96801 which compensates for temp and has a range of 0-85% Brix
Simon