I don't think I can put it better than Scott Berkun does in The cult of busy:
The person who gets a job done in one hour will seem less busy than the guy who can only do it in five. How busy a person seems is not necessarily indicative of the quality of their results. Someone who is better at something might very well seem less busy, because they are more effective. Results matter more than the time spent to achieve them.
Great post from Scott, and definitely worth reading the rest of it.
How do you stop yourself getting busy? For me, the biggest challenge has been how to learn to say No to stuff, as there's always the fear that if you say no once, you may never be asked again. Accurately judging how long something will take so that you don't take on more work than you can manage is another key trick. But I'm still doing battle with the insidious culture of overwork that insinuates itself into even the most logical brains: Finishing my day's work early means I'm effective, not lazy!