So, I got my sodastream and finally had a chance to test it out a bit. The bottle holds 1 liter of liquid or so, which looks like it would be 3-4 12 oz. bottles (too lazy to do math). Anyway, the soda stream is a very simple device. It is essentially a CO2 tank with a fancy plastic case. There are no electronics and you basically vent CO2 from the tank into the 1 liter bottle to pressurize the bottle and force carbonate the liquid. It appears to work pretty well. I might stop and buy some store cider to do a comparison test.
On thing I noticed. I only carbonated 1 bottle of cider, so there was a considerable volume of empty space in the 1 liter bottle. When I released the pressure on the bottle, the cider "foamed" and expanded considerably. I think if you tried to fill the bottle to the fill line, you could end up with a big mess on your hands. Not sure what the best amount would be, but I expect it would be half a liter or so.
From looking at the device, I don't see how carbonating something other than water could possible damage it. The only thought I had is that you would get the nozzle dirty and could breed nasty stuff on it. Nothing a little cleaning wouldn't take care of.