I use a Highgear altimeter watch and it's pretty accurate. The most that I've seen it change after a three hour ride starting, while starting and stopping at the same point is about 40'.
One thing to note about barometric altimeters is that you can be standing still and they will note an elevation gain. This is because the pressure is going up and down slightly and everytime it drops a little bit, it will show that as an elevation gain. In other words, you can be riding on a perfectly flat road and see a elevation gain.