Does doing more than one thing at once help or hurt your productivity ?

I try to save time by working on several things at once . But sometimes it seems like nothing gets done right . It is easy to lose focus when you switch between tasks , but it feels slow when you do nothing .
How do you handle this if you are in charge of more than one project ?
Do you work on one thing at a time or switch between several ?
Do you want to know what really works better ?

Read the book "The ONE thing" by Gary Keller. It will change the way you think about productivity.