most people i see mount them under a sink, and split off of the cold water line leading up to the sink. i couldn't afford a full size unit, so i bought the kent marine barebones ro unit, it was about 90 bucks. for an additional 12 dollars i bought a fitting that went onto my sink faucet that diverts water to the unit. i have it mounted on the wall neat the sink using a couple 3 inch screws into the stud. the unit weights like 2 pounds so a wall mount was easy. one line connects to the faucet, which has a pass through level thing, one line goes down the drain as waste, and one line goes to a little rubbermaid holding tank.
the unit only does 20 gallons per day, can only be used on chlorinated water systems due to an organic filter (so no well water) is rated for 200 tds water supplies or better. there is an optional attatchment that will di the water for like 75 bucks that i plan to buy in the near future.
if your needs and water supply match the specs of this system, i would recommned it. setup was complete in less than 5 minutes. for a little more though you can buy a full blown ro/di unit and do a wierd under the sink mount, running lines through the basement, multiple outputs with storage for drinking water etc.... depends on your needs.
HTH
