If I were not writing off most of the bill as part of my business I would have a hard time justifying the cost. Email/calendar/navigation/phone and tethering are business travel essentials for me. I use a DroidX, not an apple but had an iPhone before this and it was adequate for my uses as well.
The answer to the "Should I get a smart phone" is easy. Will it enable you to be $100-200 a month more productive? If the answer is yes then it is a no brainer... if the answer is no then you have to say "I'm cool with a $100 to $200 a month bill for the opportunity to play with technology" otherwise the answer is a big fat no.
Alfa... you are a runner... my only concession to app-mania is a slick app from MapMyRun that calls out pace, distance thru the headphones even while music is playing and logs your run so you can see elevation and distance, upload and share with friends, etc. Very helpful for monotony - I can set it to razz me for going slow.

