We got one of these about a week ago, along with a whole-house water
softening system. So far, everyone gives the filtered drinking water a
"thumbs way up" - no more messing with Brita filters, distilled water,
and the like. Very nice, highly recommended - the stuff tastes great,
and it's got its own tap on the kitchen sink and a 4-gallon holding tank
so you never seem to run out. The only downside is that the pour rate
from the r.o. spigot is a bit slower than from a normal tap.
Cost - I don't actually know yet. Purchased and installed by a friend
who's a plumber. Google "reverse osmosis". The water softner is pretty
nifty, too - we have to use less than half as much soap, both on
ourselves and especially in the laundry and dishwasher, as we used to.