I've been using a GPS for years now. I have found they are very accurate most of the time and never need calibrating. I can switch the GPS to any bicycle I want without having to do a thing to it. Today's ride is an example. According to my GPS, I rode 23.36 miles. I then went to my Delorme mapping software, plotting out the exact route I rode today and used their software to determine the miles. 23.27 miles the software determined what I rode. That's pretty darn accurate.
Now in a downtown area with large skyscrapers or in a dense forest environment, the GPS probably isn't going to be as accurate as a probably calibrated cyclometer. But the key is PROPERLY calibrated cyclometer, meaning you take a ruler to measure your diameter of your tire rotation and use that figure, not the generic figure that the instructions tell you.
But there isn't a rule that says you can't use both? With a GPS you can download your rides to a map or use the GPS itself as a map. Cyclometers are very inexpensive and I used both on my tour in 2005.
I wouldn't say the altitude on a non-barometric GPS is useless but it isn't nearly as accurate as a probably calibrated barometric GPS altimeter. But if you got a low pressure system coming through your area and you don't calibrate the barometric altimeter, it isn't going to be accurate either. According to the manual, a barometric GPS altimeter should be calibrated every few hours.