Once you have a smartphone, you can't live without it. Not because you get addicted to it like crack, but because it's so darn useful and versatile.
On a recent vacation, I used my HTC Thunderbolt as: a phone, a text message device, a satellite navigation unit (GPS), a web browser, an MP3 player, a game console, and a camera. Seven functions. When I started thinking about it, it can also be: a calculator, a flashlight, a notepad, a WiFi hub, a voice recorder, an FM radio, and a bubble level. I'm sure there's many more. Having all that functionality in one device in your pocket is incredible.
It's the computing equivalent of a Swiss Army knife. Get a smartphone and you'll love it.