With the internet moving so fast into the world of mobile devices, it's important to design for a variety of platforms. Many websites these days have a unique mobile optimized site which is great for usability.
Hamburger menus are everywhere. They are popular and they declutter mobile UIs but the downside is that the menu items are less discoverable and require an extra click.
Mobile device usage has been on the rise for years, driven by the increasing popularity and capabilities of smartphones and tablets. It's safe to say that, in the past years, the majority of internet users globally were accessing websites through mobile devices.
It depends on:
- Your Budget
- Usage of native API
- If you have an existing web app - in this case, it's easier to convert it to a web app by adding CSS
Note: An iPhone (or WP7) web app without a network connection, will not load the web page you were on previously.
Making a responsive website work well on a mobile is not easy but with the right tools you can save time and avoid bugs. You should use Chrome DevTools Device Mode to test different screen sizes.