I've been using iphone as my only device now for two days (my treo is plugged in, in a drawer - will cancel sprint next month).
My wife plays with her iphone when she gets home from work, but still hasn't switched over from her samsung sph-i500 palm phone.
For me, I use the iphone for both home and work (I'm currently a lawyer after a lifetime as an engineer). The great thing is for viewing word docs and pdfs in emails it blows blackberries away. It is also snappier and easier to view those docs than on my treo (which required 3p software like chatter and documents-to-go to accomplish that). Exchange activesync support would round out this functionality - right now I have to forward mail to my personal server because IT only supports blackberry - exchange 2003 support is coming shortly. To get calendar stuff in sync I use plaxo. Over-the-air PIM sync would be extra-nice.
From a useability standpoint, the only thing that does bug me about iphone is that I have to poll it. I have to turn it on and press buttons/the screen to figure out if I have received any emails. I'm getting used to that, but, as I've said, I hope they improve that.
As for my wife, she's loves finally being able to sms; she had no idea how that worked, because on her phone she can receive, but not easily send, text message (it's really old, and sprint sucked extra hard back then). She loves finally being able to read and send email (gmail via pop for her). Her main hold-up is that the phone is too gaudy, and she doesn't want to appear too snooty when she's seen with it at work, so she's waiting until our anniversary (next week) so she can at least say it was a gift and she didn't buy it herself