I started using clipless recently. Aside from difficulty clicking in when the stoplight turns green, I like riding with clipless. As others have said, it helps your feet stay put when you are spinning fast.
However the downside is that it encourages sloppy pedaling. When I was riding my single speed bike with flat (with straps) pedals, occasionally I have to spin very fast. I notice that I have to focus on my pedal rounding motion to avoid having my feet come off the pedals. Whereas with the clipless pedals that's not a concern. You just stomp on the pedal any which way you like.