Uh...just about every computer expert I've ever met or talked to has overclocked their video card.
The difference can be very apparent at high resolutions, where that extra 10+fps can make a big difference. I can play Quake 3 at 1600x1200 at 53fps, as opposed to my normal 40. Big difference in smoothness.
Roosting video cards? If you have good cooling and keep the dust off you should have no problem. In my experience, I've seen only 2 fried cards. One was my friend's defective voodoo 3, which died at an underclocked speed of 155/155. The other was another friend's tnt 2. He never cleaned off the dust in the 1 year he had that card and frequently ran that computer 24/7. The poor thing suffocated to death.
If you are pushing the card to a speed it can't handle, you generally get some signs before its death, i.e. crashes, artifacts, and/or complaints from the power company.
And so what if it cuts its life in half. Its the same with CPUs. Generally, the people that overclock are the same people that buy new hardware every 1-2 years.