I've owned a smartphone for a long time now. I purchased a Nokia N95 back in 2007 (in Finnish only, sorry), which I would consider my first smartphone. Albeit somewhat different from modern pocket computers, it did among other things have apps, a decent onboard camera, a gyroscope, a GPS-chip for pinpoint location services. Pretty much the most used features modern smartphones equip, although recent years' development have added improved usability into the mix.
One monolithic issue still remains though, which hasn't much changed since those early years; the minute you leave your home country, your phone becomes useless, save for the camera, and possibly the GPS if you have the luxury of offline maps.
Unless you're willing to endure the horror of a colossal phone bill from enabling roaming mobile data, you won't be using the routing feature in your maps app (unless you have an onboard address database, which I don't); you won't be looking up addresses for that shop you were looking for; you won't be updating social networks with your witty comments and/or stunning photographs. To top it all off, open wi-fi networks can be surprisingly hard to find, unless you happen to be in a busy area laden with cafés, restaurants or bars.
Not having an internet connection on a smartphone makes it about as useful as a wheel-less wheelchair: you can still sit comfortably, but you're not really getting anywhere.
Luckily, within the EU, there is a set rate ceiling of 0,70 euros/MB from July 1st 2013 (again in Finnish only, sorry), and a maximum charge of approximately 60 euros, after which your data connection will be terminated. Having a ceiling for data costs is an improvement, surely, but that will only give you a measly 85 MB, which is quite little considering the amount of data flowing in and out of handsets these days.
While waiting for better days for roaming data, do travellers a favour: open up your wi-fi network, even a fraction of your bandwidth is a great help.