Coming from a heavy thats usually pretty effective (with a good medic of course), I have to disagree with the people saying an ubercharged heavy is no good. If the medic runs in front of you and lets you close the distance quickly, I can usually take out 1 or 2 sentrys. The secret is killing the engi within the first 15% of the ubercharge, if the engi is running to his sentry then kill him before he gets there.
Another thing about medics, the ones who will follow you into the fray( as long as your a good heavy and protect your medic) will be the top scoring player on my team. I have went over 100+ kills with only 15 or 20 deaths when we had some good medics on the team. The best is when you have plenty of medics and one can basically be your personal medic, thats when its pwn time for me ehehhe.
I guess the secret is good coordination with the rest of the team, the heavy's wide stance soaks up a lot of fire that would otherwise be hitting the rest of the team. I have around 90 hours or so with the heavy, but the soldier is my greatest class along with demoman. In a pinch though, along with a good medic.....an ubered heavy used at the right time in a fight can turn the tides of war in a flash.