I am actually on my 3 or 4th? smart phone - Blackberry and I-phones. Had to start carrying a portable phone 24/7 due to my work back in early 80's when they were not the type you put in your pocket & not very portable ;-) Have replaced and upgraded phones ever 2 years or so since that time.
Love the smart phone as it means that when on a short 2 week trip or so I often dont carry my laptop - really not needed. If on a trip of a month or more then I might bring the lap top but use it mostly to process the photos I have taken.
There are a number of great travel apps I use a lot: Gasbuddy, Level, Flash
Light, Weather, Currency Exchange, Google Translator, FGRV, an app called Mela's Tools that gives me the conversion for lengths, weights and areas etc when I forget how to calculate the US way of doing things ;-) A few good Bird and plant apps for ID'ing them. One of my fav apps is called Star Walk - point the phone at the sky and it will tell you what stars you are looking at. Very Cool!
Regarding using a smart phone instead of a dedicated GPS all I can say is for me based on past experiences that may not be a great idea for a number of reasons:
1) I travel a lot in 2 counties (and more) & when outside my home country I need to buy a travel data plan but even with it I am still charged for data usage all be it at a reduced rate and a smart phone GPS sucks back data - you can eat up more than a dedicated GPS is worth in just a few days of use even with the lower rate travel plan. With a dedicated portable GPS you can take it to just about an developed county in the world and use it with no data plan needed. Just need to down load the map of the country you are going into the GPS before you go.
2) If you are traveling on your own having to look at your phone to push buttons is not a good thing.
Even if not on your own if you have a co pilot who is directionally challenged even with map or phone in their lap the GPS goes a long why to help keeping friendships in good standing. ;-)
2) I have found the GPS on both my Blackberry and Iphones to be more than a little wanting on many occasions. Sometimes very slow to refresh and you may find you have passed where you are going before it even shows up on the screen. The situation is the worst in areas with limited cellular coverage - in many areas you may as well throw the phone away if you were hoping to use it for GPS. The most current Iphone GPS is the worst yet - very wanting - it gets lost in downtown Vancouver. If I only had a phone GPS I would stick to the Google based app as of the two its the better but still not a great option IMHO for driving.
3) Hands free - a lot of places have laws prohibiting the use of phones while driving. You cant have the phone even in your hand. My GPS is blue tooth so it connects with my smart phone while its still in my pocket, without me having to do anything every time I get in the car. It instantly downloads my current contacts etc from the phone into the GPS - so if I want to call someone I just have to say call so and so and it does it. Or if the phone rings the GPS tells me on screen who is calling and I can answer the call using my voice only. Same with text messages they just appear on the screen. Dont need to even touch the GPS.
4) Most of the better blue tooth GPS's can be the best co-pilot you can have in so far as telling you what exit to take to the nearest gas station or grocery store by brand or even the closest state park for camping. All without having to take your hands off the wheel to push a button.
Love my smart phone but if I had to choose between taking a dedicated GPS vs a smart phone on a trip I would take the GPS every time. One can always stop at a library to check their email.
As far as I am concerned the dedicated blue tooth GPS was the best $125 I ever spent for travel.