Those who regularly multitask are the worst at multitasking:
In [a test] designed to measure how well [students] could filter out extraneous stimuli from the environment, the subjects had to look for changes in red rectangles while ignoring blue rectangles displayed on a computer monitor. Infrequent multitaskers scored well on the test, but habitual multitaskers performed poorly. The blue rectangles distracted them. In another test, the students had to recognize whether they’d seen a letter before. Both groups got equal numbers right, but the high-media-multitaskers made more mistakes. They “remembered” things they shouldn’t have, indicating-according to the research team-a diminished ability to filter irrelevant information from working memory.
You might expect, however, that frequent multitaskers proved expert at switching between tasks. After all, they get a lot of practice! But the Stanford study proved the opposite to be true. When asked to switch between classifying numbers as even or odd and classifying letters as vowels or consonants, the frequent multitaskers were slower than the low-multitasking students. The researchers suspect that the inability to filter out the previous (and now irrelevant) task may explain the slowdown.
Mind Hacks has an excellent take on this research, and BBC News produces the concise version.