Having a smartphone and using it regularly as a Smart phone for about a month now, I have come to some conclusions.
1. A smart phone needs Internet. Without Internet it's maybe only 30% smart.
2. Also we each have our specified tasks and we need Apps for that. Without appropriate Apps the smart phone loses its usability.
3. Ease of use is another parameter which determines the usability of a smart phone. If all functions are easily accessible and easily manipulated then the user can exploit it fully.
4. The UI performance is also another important factor and it influences the usability greatly. Fluid animations and clear interface contribute to greater user experience.