I like playing with my refractometer, but I haven't found it as useful as I had hoped.
I use it primarily to get into the ballpark for my OG and to check whether FG has been reached, and then verify with my hydrometer. The main problem is that the sample size is so small that it can be difficult to get a consistent reading.
As for actually calculating the FG using the MoreBeer spreadsheet, I am not so sure it works. For one thing, the initial Brix to OG calculation doesn't match my hydrometer readings, nor any of the other calculators that are out there (They use a correction factor of around 1.07, which is really high). If the initial calculation is off, I don't put too much faith into the FG estimates.