Being able to utelize a roundhouse kick effectively in a self defense situation should be a definite goal of any martial artist. Having said that, what you're striking with, how you set it up, and what targets your going for varries greatly depending on what art -style you practice. I focus primarily on the thai style kick to the outer thigh, making contact with my shin. This just seems to work best for me. I use it to close space to strike with my hands and-or look for the clinch for knee strikes and take down. It also works well to stop someone elses charge in, unless of cource you miss.
In addition, while kicking a bag or other stationary target is good to develope technique and power, being able to use it against a live, moving, and hostile person is a significantly harder thing to due, but should be your ultimate goal.
Mahalo, Jeremy