I have one (bought off ebay) that has a brix and SG scale. The SG scale generally tends to agree with my hydrometer.
I bought mine off ebay as well. The SG scale is generally 3-5 points over my hydrometer and worse as the gravities get higher. I've used it for the last 10 batches or so and the variance has been there every time.
Now, a couple things I may be doing wrong: I'm not calibrating it to the exact ambient temperature every time. That could certainly be throwing me off. And I haven't really used the Brix scale and done a proper conversion to SG to see if I get an answer closer to what my hydrometer is telling me.
I guess I've just gotten frustrated with it and maybe haven't spent the proper amount of time to get it dialed in. The only place where it would really be indispensable for me if I could rely on it is in the boil and since I've gotten my boil mechanics nailed down through trial and error I haven't been as motivated to really determine if I can get the refractometer to the point where I can rely on it.
I'll be breaking it out again for the brew this weekend so I'll try to spend more time with it and see what happens. The current discussion here has some good tips and info in it and maybe that'll help as well.