All of the bike computers use barometric pressure altimeters, which won't be nearly as accurate as your GPS. (Maybe there are some really pricy computers that don't use barometric pressure...) Barometric pressure changes with altitude, but it also changes a lot based on the current weather. You basically have to set the altitude before every ride if you want an accurate reading. I only use it for the altitude gained figure, which generally seems fairly accurate, but is ocassionally way off. If you only care about alt. gained, you don't need to set it before each ride.

One of the cool features of the Speed Zone Pro Classic Computer is the inclinometer; it estimates the gradient of the road based on how your altitude has changed over the past minute or so. It generally seems fairly accurate on longer steady climbs.