I have the Cateye Altimeter. It does not subtract for downhills either. It also keeps a running total of your climbing (until you reset the computer). I like it a lot, but will be replacing it with the Blackburn Delphi 6.0, only because it also includes an HRM and current incline.

One problem with all pressure based (as opposed to GPS based) alitmeters is it's effected by barometric pressure. I need to reset it to my "home" altitude nearly every time I take it out of the garage.