I wouldn't be without one.......... I often check during the brewing process. I use it during the mash, particularly when I'm doing a brief mash, such as my 20 minute "inline" mash where I start with hot tap water at about 130F out of the tap, dough in, rapid heat to 140 or so, then slow heat up to 155 over 20 minutes. I know where my gravity will be at the end, and I can monitor the progress of the conversion and control temp accordingly. It also works to check gravity after the mash and before the boil..... If you miss, you can add some DME, or whatever.
I don't consider them useful for FG readings, but I don't bother with FG readings unless I'm using a yeast that is known to stall, or working with very high ABV brews. With very high ABV brews, I add invert as I go along, so a refractometer would be useless.
H.W.