So I finally got the power laid to the (exterior) water tanks, installed a drain, set up the RO/DI, and I can start mixing salt water...
The MA887 is pretty cool - it needs to be calibrated first with some pure RO/DI, which sets a zero-point, and then you just place a few drops of water onto the prism (which sits in a well, to make it easy to cover the prism), and a couple of seconds later, you have a reading in PSU, ppt, or specific gravity.
It also tells you the temperature at which it takes the reading, which may be useful if your storage tanks aren't at the same temperature as your display tanks
I did a quick check after just adding some salt to a 60G tank, and starting the powerhead to mix it up - it was reading 1.009 (SG). Putting the same water into my manual refractometer gave me a fuzzy-sort-of-reading between 1.010 and 1.009, so it's at least similar. I'm not sure if the 1.009 is a good thing or a bad thing - there are no error-bars on the display

But it is quite convenient.
It's also a bit bigger than I thought from the pictures - it's about 8" by 4", in case that's an issue for anyone. Not for me - just makes it easier for my uncoordinated fingers to use

Speaking of which, all the "buttons" are water-tight membrane style, and the battery compartment screws in, so there's very little chance of water getting inside it.
All in all, I like it. I like it a lot.
Simon