Research shows multi-tasking can take more time and result in more errors than does focusing on a single task at a time.

"We know that if you have a person attending to different things at
the same time, they're not going to retain as much information as they
would if they attended to that one thing," said Nathan Bowling, an
expert in workplace psychology at Wright State University in Dayton,
Ohio.

Yes I agree.  I think multi-tasking increase the chance of error which means you end up taking more time than if you just did one thing at a time.  I also think it leads to a poor professional perception.  There is no doubt you can tell when someone has sent an email from a Blackberry when say in meeting or on the phone.  Full of mistakes and just does not look good and professional.

Addicted ... emailing and texting during meetings is an office no-no 

And guys having "sent from my Blackberry" in your email signature is an indication to me that you have not taken time over the response and given the focus that I expect.    Does not impress me that you sent an email from you Blackberry at 10:00PM on Friday night.  Was most likely sent from a bar anyway.