I've admitted that I am a luddite (really, just a cynic about the brightest, flashiest new thing) here and here and...
It will come as no surprise that I was an extremely late smartphone adopter. I was essentially forced to get a smartphone because I was running field projects and it became untenable to not receive and respond to e-mails immediately. Running back to the trailer a couple times a day to refresh the laptop's e-mail wasn't working anymore.
Everyone around me swore that my smartphone would be life-changing. And I do use several functions in the field: the stopwatch. The calculator (although my dumbphone also had a perfectly adequate calculator). The alarm clock, with its ability to set different times for different days. The ability to text reasonably quickly. E-mail and internet. In a pinch, navigation and camera, although I have dedicated gadgets that work much better.
I haven't really added any applications, although I probably should invest in a few that would be especially handy: a flashlight (I've got a pretty good one on my keychain already), a magnetic detector/stud finder (although I don't think it's very sensitive), and a more accurate location finder/compass. I've focused most of my efforts into getting a bulletproof, water resistant phone with a good battery, so that it will survive tough conditions even if it's not very high-powered.
I do have to watch (in myself and in others) that smartphone doesn't become all-consuming. Just because you can browse the internet while you're supposed to be overseeing a subcontractor doesn't mean you should. And just because you get e-mails fired off from a bunch of night owls doesn't mean that you should feel compelled to respond at all hours, either. I really try to be on when I'm supposed to be and off when I can be, but the smartphone makes it easier to blur those lines.