I agree with the idea of having goal-line cameras, but I think we should resist the temptation of using the technology for things like offsides and contentious penalties. At the end of the day, somebody will have to look at a video and make a decision. You can't remove the possibility of an error because it's still somebody's opinion.
Do people really think that everybody will suddenly start agreeing with every decision just because video evidence is being used? For every 100 people who think a certain incident should have been a penalty or a red card, you can bet there'll be another hundred who'll think it was a dive.
If the decision is a cut and dried one like whether or not a ball has crossed the line - fine. If it's anything arguable, where human error can still become a factor, then what's the point?