li... its taken me about a year to build up to being able to stand up a hill for a minute... and thats a moderate hill...

It is a fab workout, because you are actually demanding much more of your body when you climb standing, than if you stay seated... just watch your heart rate...

It is also great to train your body to recover more quickly - when you sit down, you often just want to stop/slow pedaling... but if you pretend you have been chasing Lance or Floyd, or Sarah or Onone, up a hill and when you get to the top you have to sit and thrash the pedals because you know that the top of the hill is where they will attack because everyone else is tired from climbing... well, if you can put the power in when you sit, its great practice for your heart and lungs and legs to respond to your demands...

(although my perceived power when I sit and try to "power" away, and my actual power/speed dont really correspond yet but I am working on it...)