If it's calibrated and the proper equations are used, it's pretty much fine. Fine enough, in fact, that if you have both a hydrometer AND refrac, you should be able to work backwards and actually even calculate the OG.
There's plenty of other reasons to get a refrac though. Not to mention, if you're using a hydrometer only to measure FG (like many people do), then, UNLIKE most people (for whatever reason), you can get one of those hydrometers that have a much more narrow range (eg 1.000-1.030), which allows you to make a MUCH more precise FG reading than an all-purpose hydrometer... you can literally measure something every 0.0005 (eg 1.0105) with the same accuracy you'd be able to measure every 0.002 on a normal hydrometer.
Hadn't thought of that. I do like to measure both -but from time to time WILL forget to measure the starting gravity -and truthfully, my primary use for one is to assure myself that no more fermentation is taking place.