AFAIK, they seem like Auto-crafting tables but it "remembers" multiple recipes? I dunno, haven't had a reason to fiddle with one yet.
I did mess around with the Multi-tank structures in Creative, though. Apparently they seem to defy some internal logic of multistructures, since they can be composed of pratically most Xyblocks you can make (except for viewing glass, but that works in the middle parts. Must test the lamps.) They do have a theoretical limit of just above 1,000 buckets--I tried a megastructure to test that, and that was the limit I hit. And all the tank data is stored in the Valve Block, so you can do interesting things like merge multiple tanks that contain the same liquid. I need to do more testing with multitanks, but this is a hugely useful alternative to Railcraft tanks and aren't prohibitively expensive if you just want to toss up some liquid storage.
Note: each empty cube in a hollow tank is worth 16 buckets, or 16,000 in whatever measure it's in, so don't be misled by the numbers. Feel free to crunch some theoretical numbers for fun.