Mobile units are great for social purposes and catching up on the news. As far as work goes, they have a long way to go. It takes me a minute to read and answer an email on my mobile unit that would take me about 15 seconds on my desktop. No comparison.
I'd say they are somewhat counter-productive during work hours. People at meetings now play with their devices instead of paying attention.
After work is a different story. These devices have managed to make us available while on vacation and well into the evening hours. They've been great at intruding into our after-work lives. It's like being constantly tethered to your job.