With more people accessing websites using their smartphones, making sure you website is readable on a phone is not something you can ignore. Load times, readability and images often render differently on a smaller screen. If a website is less functional when viewed on a phone versus a regular PC or laptop, many mobile users will simply leave a site. According to KISSmetrics, about 20% of all web traffic comes from a mobile device. Furthermore about 50% of local searches is done on a mobile. This is critical because this is the kind of search where people are looking for you or your business while they are out and about, in other words shopping!
Google has an interesting tool to help you evaluate the effectiveness of your mobile site. You can find it here. Load in your site, answer a few questions and Google will give you some tips on how to make your site better. Best of all the tips are free!