To check the gravity prior to fermentation, it's an excellent tool. I use mine with every batch (to get the OG as well as check other stages, at times). Once you get fermentation going, you'll need to adjust the readings (there's software/spreadsheets that will help you there) to get the correct reading from it. Personally, I use a hydrometer for the FG reading so it's a non-issue.
I have used it to get gravity readings on things like cyder, apple juice, as well as check the OG for my current batches of mead (the ones that don't exceed it's scale at least). With how small a sample you need for a refractometer, I think it's a very worthy tool addition. Plus, you don't need to worry about particulate fudging up the reading.
Get one with automatic temperature compensation, as well as both SG and brix scales and you'll be fine.